After reading this question here about maneuvers within attacks of opportunity and its chosen answer, as well as this rule from the prd:
When performing a combat maneuver, you must use an action appropriate to the maneuver you are attempting to perform. While many combat maneuvers can be performed as part of an attack action, full-attack action, or attack of opportunity (in place of a melee attack), others require a specific action. Source (Emphasis mine)
as well as the rules quotes within this question, I have concluded that it is okay, by RAW (and perhaps even RAI), to deliver a spell through these specific combat maneuvers when using the Spellstrike class feature.
In short I have come to this conclusion because of the specific wording of both the rules for these maneuvers as well as the bolded text in the above quote. Spellstrike grants a free attack (read: a free attack action) and all three questioned maneuvers allow you to perform them in place of an attack action. They do not specify that they take their own action (i.e. their own standard action) and indeed do the opposite, being made in place of the attack. Therefore these maneuvers may be made in place of the attack action granted by Spellstrike and used to deliver touch spells.
There is also this post on the official FAQ from a designer on the Pathfinder team:
Disarm, sunder, and trip are normally the only kinds of combat maneuvers in which you’re actually using a weapon (natural weapons and unarmed strikes are considered weapons for this purpose) to perform the maneuver, and therefore the weapon’s bonuses (enhancement bonuses, feats such as Weapon Focus, fighter weapon training, and so on) apply to the roll. Source
Therefore if you are applying the bonus from the weapon to the roll, you are using it in the maneuver. This is further evidence that one should be allowed to use these maneuvers within Spellstrike.
Another post from the Core Rulebook FAQ says this about Sunder attempts:
The text is a little unclear here. Instead of saying "as part of an attack action in place of a melee attack", the text should read "in place of a melee attack" Source
Even though the question within the FAQ is about multiple sunder attempts during a full-round attack action, it erratas the ambiguous wording of sunder RAW which in turn supports swapping it for any melee attack, not just those that are part of (previously ill-defined) 'attack actions.'
And this thread on the Paizo forums discusses how many players build a Magus this way.
As to breaking the action economy of the game, I used to think that this was not allowable as well. Technically, though, this is exactly what Spellstrike, and the Magus, are designed to do.
Normally when a caster casts a spell with the range of touch, the caster is granted a free melee touch attack with which to deliver the spell. This is a free attack action granted by the casting, as is evidenced by this rule:
In the same round that you cast the spell, you may also touch (or attempt to touch) as a free action.
Touch Attacks: Touching an opponent with a touch spell is considered to be an armed attack
The touch attack as performed by a normal caster (i.e Wizard or Sorcerer) doesn't do any extra damage. It is simply there to deliver the spell. A caster could also hold the charge.
Enter the Magus, with Spellstrike.
The Spellstrike class feature is designed to add an extra effect to the casting of a spell, namely weapon damage. This is a, no the, key feature of the Magus; it is designed to combine attacking with spell delivery. So if you are already getting an extra effect on your action economy for the turn (weapon damage and all its modifiers, including feats like power-attack if you so choose), there is absolutely no break in the economy to allow one of the three combat maneuvers above instead of the weapon damage.
For the Magus, Spellstrike makes touch spells look either like this:
Weapon damage (+plus all modifers and/or feats applicable) + Spell effect (not necessarily damage)
or like this, as I am proposing:
Maneuver effect (Trip, Sunder, Disarm) + Spell effect
Spellstrike is designed to combine both actions and is a purposeful exception to the normal rules governing touch attacks.
Note: Dispelling a charge using a sunder maneuver treats the object as the target of the spell, while trip and disarm would target the creature as normal.