It is not recommended. Anything confusing is dangerous, and this is
confusing.

However, if you lose the 30a breaker, it is possible. You cannot use a 30a
breaker because the 120v circuit will not be rated for 30a. You must get a
20a breaker and use #12 for all of it. (I am assuming #12 is adequate,
which might not be true if your run is too long and the current is too high;
you have to check that first)

If 20a will cover the DC and whatever you want to also run on it, you can do
it. Otherwise you can't.

Overall it is a better idea to run two circuits. Did anyone mention that
you will need 12/3 wire for the combination circuit but only 12/2 for the
separate circuits? It is probably just as easy to run two 12/2 as one 12/3.

That was funny about not being able to change DC to AC.