Is it a good idea to use the word 'parlance' to mean one's area of interest in a general way? Reliable sources suggest that the word be used only where the is area is more linguistic in nature. Has the word acquired at least a bit more figurative sense in the course of time?
No. Parlance does not in any way mean "area of interest."
"Parlance," used today, ordinary refers to the vocabulary and special definitions that are particular to a group. The group might indeed be defined by their area of interest. For example, you could say:
In gambling parlance, "call" means "to match someone's bet".
This said, your parlance could be the product of any number of identities. You might refer to "teenage parlance" or "milliary parlance." But parlance is a feature of those groups, not the groups themselves. You wouldn't say "my parlance is biology," for example.
Less commonly, "parlance" is synonymous with speech in general. You and I might decide that parlance was preferable to silence if we were waiting at a bus stop, for example. The word comes to English from French, so the classic connotation would be diplomatic speech. For example, an ideal leader could be said to be skilled in arms but prefer parlance.