First, the relevant rules text (portion most pertinent here copied for convenience):
Creatures immune to level drain, whether by innate nature or magical protection, can't use the Trueforge.
I recently gave some thought to the question: which magic item can a player craft that is arguably the best case scenario for the dangerous effect? After little thought, I came up with -- Golem. (If somebody has a better idea, I'd be happy to hear it!)
An early disclaimer: the trueforge is an artifact, so access will be limited to rewards/story/plot driven by GMs. Thus, for the sake of this discussion, I ask a reply to assume that the GM is ok with giving access to this artifact in some capacity.
Second disclaimer: Crafting Golems (and really, any magic item) requires GM approval again. Let us presume, for the purpose of discussion, that a given GM is alright with the idea of a player crafting any construct in the rulebooks -- even statting out modifications from Ultimate Magic.
The rules tell the player that their fate after being absorbed into the crafted item is entirely in the hands of the GM. As a GM, what would be an appropriate response to this?
My first reaction: probably too powerful. The HD, abilities, and innate protections probably outweigh the negatives -- lose CON to HP, gain a particular weakness to a specific magic, regular healing no longer applies, etc -- thus the GM should make the PC an effective NPC, gut the class abilities, and make the player re roll.
My second reaction was: If the crafting had gone smoothly, the PC would have had their own character *and* the golem. Separately, they would have been weaker individually, but they would have double the action economy. This can be worth more in a fight than a character who has better stats/better defenses. Thus, the GM could let the player essentially possess the golem (like magic jar/possess object, only permanent)
Letting the player keep their abilities seems like an extremely niche way for any crafter to emulate being a synthesist summoner... Except the suit will cost a bunch of the player's wealth by level. However, I'm not totally positive on giving a new way for a player to hijack abilities from other classes.
What do you think?
I recommend a colossus instead.
I had given some thought to those, but they suffer even more than most golems from an inability to explore small areas.
If you're looking at a campaign that will allow gargantuan/colossal players to contribute regularly, then those are probably better.
There'd also be some sort of odd roleplay to be had for an intelligent stone colossus when in its castle form.