Why not just "spells you cast have convoke"? I don't think this template works

Thinking about it some more, I'm not a huge fan of it locking you into having to cast everything via convoke. What if it granted convoke to the subset of cards you're trying to support, and had the "no mana" restriction for another (overlapping) subset? Something like "Creature spells you cast have convoke. You can't spend mana to cast non-white spells." feels very in-flavor

Regarding template, I think "spells you cast have Convoke" makes them only have Convoke while on the stack, so they don't have it until after you've cast them. I based it off of the templating Vedalken Orrery uses to give spells Flash, though double checking it it seems it should be "as though" rather than "as if."

As for your second thought... I feel like "You can't spend mana to cast non-white spells" changes it too much -- the current version alters how you play the game, while the latter just encourages you to play mono-white. While both of them are fine as drawbacks (And I do kind of want to make a card with something like the latter ability now), they are very different sorts of drawbacks.

As for making it only work for a subset... I feel like it'd work best as either creature or non-creature, rather than something more specific. Maybe something like this? I feel like it loses a little something in the process, but it's also a lot less likely you'll lock yourself out of the game by accident.

­Chief Engineer. Flash is different, since it's a timing restriction, so you need it before you start casting the spell. No cards actually grant flash anymore, because that doesn't work. Alternate costs don't kick in until after you've started casting the spell, so it's fine to grant the ability

...I completely forgot that card existed. I probably should have actually checked for proper templates instead of going off memory, huh.

Anyway, fixed. Thanks for the correction!