My question: Assuming that someone puts together a GPS database
(similar to Garmin, etc.) for the iPhone, what would the
validity/legality of it be to having it in an aircraft? Would that
satisfy the requirement for being /G or /L equipped? Just wondering, as
that could be an inexpensive way tohave the equipment in hand/onboard.

No.

Handheld devices do not meet the requirements.

Guidance on this can be found in the AC's on the faa website.