If the target requires movement to another permanent slot (weapons and shields are also transferred from their special slots to a regular permanent slot) and all available permanent slots are full, then Touch of Midas can't be used.
I think the best behavior should be the same when you use steal and you have all permanent slots filled, for consistency.
I believe (i'm not sure) that in that case Steal behave like a Deflagration: destroy on his side while not creating on your side.
I think that Touch of Midas should behave the same: destroy a permanent without creating the Golden Relic. After all, filling all opponent permanent slots could become a strategy, exactly like filling creature slots (see Aflatoxin!).
Sidenote: i somehow forgot how useful and balanced this card is. And i never considered using it on my own permanents to accelerate drawing.
I love versatile cards.