Applying a Spell Oil from spellbladesnext to any melee weapon turns it into a spell catalyst. However, they remain ineligible for spellpower enchantments.

I thought I'd work around the issue by changing the requirement in the config from

"requires": "MAGICAL_WEAPON",
to
"requires": "WEAPON",

but it seems this is not a valid value. Is there a way around this currently?

This doesn't seem to be a bug of Spell Power Attributes.
Spell Power Attributes is a low level API, hence it is incorrect to add supporting into SPA for high level mods, such as the mentioned one.