There's a Connector API available for the Informatica platform; here you can write your own connectors using Java code.
There should be examples and documentation available, I just don't know where, I've never taken a close look at it (because it can't be used for PowerCenter which is my area). It might well be that this doc and samples are part of the client installation, but that's just a guess.
I'm not trying to write a connector so something can connect to Informatica. I'm wondering how to use another application's api as a source in a mapping.
Assuming the other application's API can be called from Java code, I think the Java Tx may be the only suitable option.
you cannot use a web service consumer transformation as the source for a DQ mapping.
You would have to read a single value out of a flat file or relational source (this can be completely unrelated) and feed that value into the web service consumer or java transformation where it can be ignored.