Me, I added a few debugging dips to the CD-i driver so that it would be easier for people to rip graphics from the game, so I don't see a problem with what Robbbert's doing.
While it can be argued that it "clutters up" a driver, you said yourself that the drivers can already decode and display the characters properly, so that implies that the drivers are already largely complete. In the event that they are not, throwing in a few more blocks of code isn't going to harm anything.
While you claim that it's "a developer aid, not a user feature," users already view F4 as a user feature, and you as a MAME dev should know quite well how much official proclamations from devs have an impact on what users think. At this point, I'd say we'd might as well run with it.
About the only suggestion I have, not having seen the code that Robbbert is adding in (the above "few more blocks of code" was an assumption), is that it should be commented as being related to the F4 viewer and not the actual functional emulation of the system.