It would not work. While you might cast a spell into it with no restrictions, the glyph seal has the following language:
An activated glyph seal functions as the spell glyph function of a glyph of warding and can be detected and disabled as a magic trap (DC 30).
So the glyph seal cannot activate unless the spell cast into it is harmful.
From glyph of warding:
Spell Glyph
You can store any harmful spell of 3rd level or lower that you know. All level-dependent features of the spell are based on your caster level at the time of casting the glyph. If the spell has a target, it targets the intruder. If the spell has an area or an amorphous effect the area or effect is centered on the intruder. If the spell summons creatures, they appear as close as possible to the intruder and attack. Saving throws and spell resistance operate as normal, except that the DC is based on the level of the spell stored in the glyph.
Additionally, glyph seal contains this language:
You can safely transport a keyed glyph seal until you activate it by pressing the seal to the desired surface.
So at the very least, RAI is that the effect is harmful. I can understand an "anything goes" interpretation, but this looks more like the item is a reusable, versatile glyph of warding than a super-contingency for free.
"Safely" is a powerful word in this item's description. If the word "harmful" is added to the items description: "any [harmful] arcane or divine spell ..." then there are no contradictions or questions about this item's intended function. The reason for that is because the entire rest of the description very strongly implies that the intent is for this to function as a reusable glyph of warding item that can be used with both arcane and divine spells. Since "harmful" has been left out of the description, the item implies its intent but does leave the loophole open. At my table, we would simply look at the cost and limitations (i.e. Dispel-vulnerable) for crafted contingent spells and know that this item was never intended to be used as such.
Full analysis:
A glyph seal allows you to convert any arcane or divine spell of up to 2nd level ...
The crux of the issue. This statement taken alone definitely allows the use of a glyph seal as general contingency spell item ...
... into a symbol similar to a glyph of warding.
... but as the question points out, this is the first clue that the item is supposed to harm an unsuspecting victim.
To do so, you must cast the spell while holding the seal; doing this is called keying the seal.
(How to set the glyph seal's spell.)
You can safely transport a keyed glyph seal until you activate it by pressing the seal to the desired surface.
Second clue that this is not intended to be helpful, as it specifically claims that the glyph seal can be safely transported which implies it is dangerous.
The seal can adhere to any nonmagical, nonliving object.
(How to activate the glyph seal.)
An activated glyph seal functions as the spell glyph function of a glyph of warding and can be detected and disabled as a magic trap (DC 30).
This section very clearly refers to the glyph seal as a trap with the same function as the spell glyph function on glyph of warding -- this is not something intended to be used as a buffing mechanism on a knowing creature.
A glyph seal is unaffected by the spell within it.
This refers to the upcoming destructive -- and possibly AOE -- nature of what the glyph seal is intended to hold.
Once activated, the now-empty glyph seal can be retrieved (though it requires a successful DC 30 Search check to find) and used.
(How to recover a glyph seal)
You can always remove any glyph seal you keyed and activated to redeploy it elsewhere.
Note that this section might make it actually impossible for the person who keyed a glyph seal to set it off. Why? Because if the seal is triggered by opening a bag inside which the seal is affixed, then the person who keyed it is incapable of triggering it because one is always capable of removing any glyph seal one activated.
The evidence here is overwhelming that this item is not intended to be used as a buffing mechanism. It is intended to be a trap item.
Additionally, let's look at the next closest item to doing what this thing can do:
Craft contingent spell (Complete Arcane p.77/p.139):
- You can only have a number of crafted contingent spells up to your
hit dice, but there is no restriction for glyph seal
- Crafted contingent spells are destroyed by dispel magic, glyph seals
are not
- A level 3 crafted contingent spell costs 1500 gold (lvl 5)/6000 gold
(lvl 20), glyph seal costs 1000 in either case.
- A level 5 crafted contingent spell costs 4500 gold (lvl 9)/10000
gold (lvl 20), a greater glyph seal costs 4000 in either case.
- A crafted contingent spell is single-use, whereas a glyph seal can be used
repeatedly.
If someone argues that the glyph seal items are intended to completely usurp the already-too-powerful contingent spell items, they are simply ignoring all the evidence to the contrary. Their intended use is quite clear if someone accepts that an author simply forget to add "harmful" when they were reducing the spell levels from 3:2 and from 6:5 so that the item wasn't quite as powerful as a cast glyph of warding. My table has ruled as such, and we firmly believe that the RAI for these items is to be traps that are slightly less powerful than a cast glyph o warding but usable by anyone, not hyper-abusable buff items.