Giving this one a good bump. I recently got on the MPE train with Seaboard block. While editing the other CC is as nice as it gets in Reaper, with the help of "select CC with Notes" (insert the usual "this could be more comfortable" caveats here), editing pitch bend data is practically impossible, because
a) as the OP points out, the data is taking place in a marginal visusal range, so that a semitone bend makes little visual difference in the CC lane even when the lane height is maxed out. Don't even try to see a finger vibrato if you like to keep some eyesight for later use.
b) there is no aid as to which value you'd need to bend to straight pitch values.
To illustrate, here is a snippet of Reaper's pitch bend lane fully expanded vertically in a maximized MIDI editor window. (Because of forum attachment restrictions this only shows the area around the center line, it is as zoomed in as you can get on my screen):
You can tell that some note goes first up a bit and then bends down to a bit lower than the original pitch. Editing this is bound to fail. (And this is just a single note. I leave to your imagination how lost I am when looking at chords with bends or vibrato that would need fixes.)
To help with issue a) a vertical zoom as proposed would do, but for solving problem b) Reaper would need to be aware of the pitch bend range the input device is supposed to send (that would be 48 semitones as per default on ROLI devices, but can and must be possible to be changed. EG Omnisphere can bend 24 semitones max and for Kontakt it's only 12 semitones. Given such an option Reaper could help with horizontal guidelines at straight tones.
Bitwig (and probably other DAWs too) has this part covered nicely by overlaying the pitch bend data to the notes area of the piano roll, so you can see at a glance which note bends to which and where it does a vibrato (and can edit those curves right there). That way the user doesn't need to think about channels at all when editing MPE pitch bend.
The (almost) same musical snippet looks like this in BW:
Here you can see very clearly what is happening, even the slight vibrato that I played is visible and editable. Plus I can tell that the downward bend is ending up slightly sharp and go in and fix it with an easy select and drag edit. It's equally easy to change the bends to reach any other interval while keeping the transitions neat and smooth if I am so inclined.
I believe that this is the direction to think about for Reaper as well. While Bitwig only has this for pitch data I think it would be worth the effort to go further than that and make other controller data visible in the notes area too (thinking Cubase's Note Expression here)
Playing MPE stuff is heaps of fun, and with time there will be more devices sending that data, now that it's officially in the MIDI specs. I am very happy that Reaper is pretty good at dealing with the data - apart from pitch bend. Please consider putting in some effort here.