I went and puttered around the trac source, and eventually found something that the comments make sound like the right thing.
https://netvor.tk/trac/arcomage/browser/dev/inc_refactor/core/Db/Game.php#L707
It looks kinda like you could actually copy a function very much like that for the card effect, except instead get it to disable targeting, use the chosen card instead of $card, then after that override the stuff involved with summon/production (lack of keyword auto-handled!); if so, there would be a bunch of bugs, probably involving it getting confused about which card position should be used for what, but they seem fixable without horrible hacks.
Also, whether I can use "play the targeted card in hand" would depend on exact implementation! Maybe summoning can be allowed to make the card dangerous to play sometimes (cause it would get rid of itself)? Or maybe it doesn't need to be that way...I was thinking that for Conscript army, Far Sight and other similar card position stuff, it would be using the position of the Workshop instead of the chosen card, but that was just an odd assumption.
Edit: Somehow forgot one of the blocked things.