If GGG thinks that crafting gg items will be too easy with this method, maybe restrict it to items of that same base type, similar to assembling enough items for combining in Synthesis.

This restriction wouldn't matter at all. The gg crafting scenario here start like this:

1) Craft the usual mirror-tier item with 5xT1 and an open affix slot. 2) Alt-regal a 2-3 mod rare of the same base that includes the sixth T1 mod you want. 3) Turn that into an Oblivion Orb.

At this point, it depends whether the Orb actually displays the mod you'll end up with when you use it. (You didn't make that clear in the post).

If it does, then you have just obsolete'd Eternal Orbs for this kind of extremely high-end crafting, as you essentially have a guaranteed exalt slam. If it doesn't, then eternals are still useful but much less so, as you can have a 50% chance to get the exact T1 mod you want.