I am aware that there was already a feature for this in an earlier version of Hookpad and it was probably removed for a good reason, but I want to propose a new approach to this topic.
I agree that arbitrary slash chords are an issue due to the fact that there is no universally accepted way of notating them in roman numerals. But you barely see chords like C/F# anyway. 90% of the slash chords I encounter in my music analysis endeavours are part of a pedal point progression. It is extremely common to see a simple progression like I - IV - V - I with a tonic pedal in the bass. You find those in almost every genre of music.
Unfortunately, transcribing a progression like this in Hookpad will leave you with utterly absurd and useless chord lables such as I7sus2sus4 (V7 over tonic pedal). The worst part about this is that all chords that don’t contain the first scale degree will be labled with tonic function, which is obviously insanely absurd.
Here is an example of what I - IV - V - I over tonic pedal would currently look like in C major:
Luckily there is a very simple solution to this issue. Due to the fact that every chord has the same bass note in a pedal point progression, we can just omit the figured bass and use root position roman numerals for all chords within the pedal point progression.
That could look something like this:
The colors in the chord staff already indicate the scale degree of the bass note, but to avoid any potential confusion one could also add a little sign that says “Tonic Pedal” or something like that.
I think a feature like this would be a very worthwhile addition to Hookpad since these types of progressions are extremely common in almost every genre of music.