Or as another example - a wizard and a cleric cooperate to create a scroll of heal. The wizard is the "creator" and thus gives up his xp. Is the scroll arcane or divine? The RAW states it is arcane - which means that neither character can use it without using UMD.
Actually, RAW states that it is divine. The end result looks at the spellcaster contributing the spell to be cast, not the crafter. I believe it is covered either in the FAQ or the DMG. I will try to locate it and post it later.
Unfortuneately not everything in D&D "makes sense" as we have discovered repeatedly when looking at the RAW. That is one of the main reasons for people using house-rules, so that the rules they use make sense to them.
Which I feel can only end up over-complicating this debate. What we should be focusing on is answering the OP's query solely from a rules perspective, without inserting our own POV about how we feel the rule ought to work instead. Let him houserule his way if he does not like the final answer, rather than we doing the houseruling and trying to pass it off as canon.
For example, the warlock cannot take supernatural transformation since his abilities are technically derived from his class, even though his flavour suggests that his powers are "innate". You can argue until the cows come home about why he should be allowed to do so, but the fact remains that he cannot by a literal reading of the rules. If he is not happy about it, he can certainly rule otherwise. But we, as rules interpreters, should not claim the latter as RAW simply because we feel it "ought" to be the case.
There is no such thing as a caster that is neither arcane nor divine other than a campaign specific one. None of the Core nor any of the supplemental books addresses such a case at all. That is one of the important pieces of evidence to consider.
And the warlock is technically not a caster in the sense of the word. For instance, it does not benefit from practiced spellcaster (as clarified by the FAQ). It is merely treated as a spellcaster for all intents and purposes of interacting from a very narrow and specific list of benefits (all of which are scattered throughout CA), such as what prcs it may qualify for, what feats it can take and the like.
Here is the relevant entry (page27 for reference).
What effect would the Practiced Spellcaster feat (Complete Arcane, page 82) have on a warlock?
A strict reading of the feat’s benefit indicates that the warlock would gain no benefit from Practiced Spellcaster. The warlock is not a spellcasting class for normal purposes—the exception noted on page 18 of Complete Arcane applies only to prestige class benefits—and thus it could not be selected as the class to be affected by this feat.
This means that it is in fact, neither arcane or divine. Why it was included in complete arcane, I have no idea (perhaps the designers felt it covered a role similar to other arcane spellcasting classes?), nor do I feel it really matters in this context.
As such, my reply is, and has always been this: The warlock can emulate spells to meet the crafting requirements of magic items, and the nature of those spells (ie: arcane or divine) will depend on which he wants to replicate (typically, you will want to choose the lower lv version or the version without costly arcane material components to cut down on crafting costs). For instance, if the warlock12/chameleon2 wishes to scribe a scroll of identify, he can (and should) opt to emulate it as an oracle domain1 spell, rather than the wizard/sorc version (since the divine version does not require a 100gp pearl, and is thus that much cheaper).
