Virtual feeds don’t actually exist per se (hence the “virtual”) the data is created on demand to answer a specific query (eg api call) if the question isn’t asked, the answer isn’t given and there is no feed data.
You could do that, phptimeseries and mysql feed types have been on the wish list for many years so maybe don’t hold your breath.
When the myelectric app was originally created @TrystanLea made the decision to only allow phpfina feeds to be used. The apps module has under gone several revisions but that feature remains.
There are many discussions about those limitations but still only phpfina feeds are supported. Most feed types seem to work perfectly well with the apps module, but beware the virtual feeds can be quite resource demanding (especially on a Pi), but virtual feeds are specifically designed with graphing in mind.
Whilst a config setting so users can choose what feeds are supported could be a reasonable compromise, currently the only way to include other feed types in myelectric/mysolar is to edit your local code (see “Does emoncms expects data to be send reguarly? - #2 by pb66” for info on how to add other engine types).
Note that making local changes will of course block future updates via git pull unless you use git stash before pulling in the updates.