When the Dungeon Master's Guide says, "A single weapon cannot have a modified bonus (enhancement bonus plus special ability bonus equivalents) higher than +10" (221), the Guide's discussing a magic weapon that's created with a modified bonus higher than +10, not limiting an already magic weapon's capacity to be further temporarily magicked beyond an effective +10 bonus.
The Magic Item Compendium clarifies that
A single weapon cannot have a modified bonus (enhancement bonus plus property bonus equivalents) higher than +10, nor can it have a market price (not counting special materials or the price of the masterwork weapon itself) of greater than 200,000 gp (or 200,000 gp for each end of a double weapon). For weapons that exceed these limits, see Epic Level Handbook. (221)
(Emphasis mine.) This forces the reader to the Epic Level Handbook, which says
There is no limit to an epic magic weapon’s enhancement bonus, to the market price modifier of an epic magic weapon special ability, or to the total of an epic magic weapon’s enhancement bonus and market price modifier. (129)
So a magic weapon can even be created to have an astronomical effective bonus if a creator is willing to pay an astronomical cost, and, by extension, temporary magics pretty much don't count at all for any of this because temporary magics don't increase a weapon's gp value. (Disregarding the possibility of a con artist duping a mark into thinking a temporarily magicked weapon is an epic weapon, of course—that's not a game mechanic but a plot.)
Admittedly, the Dungeon Master's Guide should've been more up front that it's all about creating weapons not casting spells on weapons rather than making the reader bounce from book to book to confirm.
For example, this means a +1 briliant energy vorpal longsword (hence possessing a modified bonus of +10) can benefit from a Wizard 20 casting on the weapon the 3rd-level Sor/Wiz spell greater magic weapon [trans] (PH 251-2) so as to grant the weapon instead a +5 magical enhancement bonus, and the weapon can also benefit (although not much) from a level 20 cleric casting on it the 4th-level Clr spell undead bane weapon [trans] (Spell Compendium 226), even though, combined, these spells would make the +1 briliant energy vorpal longsword effectively a +5 undeadbane briliant energy vorpal longsword (a weapon with an effective bonus of +15).
Looking at things from another angle
One of the game's centerpieces is that a specific rule can override a general rule. This allows the rules of the specific spell greater magic weapon to override the broader, general rules about magic weapons presented in the Dungeon Master's Guide, essentially allowing the spell greater magic weapon to function as per the Player's Handbook on weapons that already possess a magical enhancement bonus, magic weapon special abilities, or both so long as one keeps in mind the rules for stacking bonuses.