Love your work guys. I am no legal expert either, but as long as Steem Engine is not selling tokens or offering any means for selling tokens, then I would think that you're absolved of any legal implication in the use of user-created tokens. You're providing a means, but not an end (that's for the end user).

I don't see how the challenges you're facing are going to be any different than they will for Steemit Inc. I think in the case of Smart Media Tokens because STINC is a privately run company, they might have a little more legal tape to navigate in regards to token issuance.

Getting legal advice is smart, but I think you're fine as long as you have clearly defined terms of service and do not encourage the creation of potential securities (being sold or traded for any measurement of currency).

I really want to create a Steem Engine token myself, once I can scrounge up the 100 STEEM required to do so. I have a few dApps I am working on, one of which would really benefit from a custom token.