P.S. there seems to be a problem with some of the hybrid mark image links. I will update this post with the hybrid mark symbol once they get fixed.
Does http://dl.dropbox.com/u/33829445/ElementsCustomMarks/TeriesMark.png work for you?
Awesome!
Updating OP now.
Does the target gain the stats as addition or as replacement? e.g if you have a Skyforge Scrap-golem out with 2|6 and you replace a Titan, will the golem be 7|50 or 9|56?
It gains them as an addition.
So, yes, replacing a titan could yield a very massive creature. (9|56 in your example)
The limitation on targeting is there to help prevent just that kind of abuse.
For instance, otyughs and scarabs are not golems or weapons so you can't use this to make a 50HP devourer (unless you get a really lucky mutation roll or something)
The catapult combo could be quite potent, but its still requiring you to pack multiple titans in your deck since you need to replace a titan every time you want to transfer its HPs.
Plus, unlike the animate weapon combo, it relies on a creature to do the stat transfer. This lowers the card count but makes it more vulnerable to CC so it should balance out.