I do not agree with a general population cell system for both the Citadel and the Gendarmerie. It creates a number of issues, and clashes with themes in Port-à-Lucine in specific to do so. However, in the Citadel, you can create prison roleplay by putting prisoners in cells near eachother instead of on opposite wings of the Citadel. It's easier to seek reform that way than to redesign the prison system.
In the Gendarmerie, I have suggested multiple times over the years to see the cell doors changed from LoS blocking ones to open cell doors so prisoners down, beside, or across the hall could speak to eachother during their sentencing. In PaL in specific, the prisoners need to be separated because the Nobles who are in the cells at any given point are afforded exceedingly high privileges, such as cots, tables, writing implements and exclusivity of their prison space.
Multiple prisoners can be put in a cell as well, which has been done before, however it often results in something occurring that negatively impacts or reflects upon the Gendarme PCs who are in-charge of watching over them.
I'd sooner look into enforcing standing orders for the Garda to put prisoners beside eachother (unless they lose their privileges of having a neighbour for some reason) than I would try to rebuild the cell system. Keep in mind, if you wanted to log in and RP with yourself in your cell, that's fine. And usually, Guards will facilitate visitation or they will arrive to feed your character/etc when they can make time. However, this being said, being in prison in real life can be similarly boring. It's not intended to be a reward system, and there are few fates more dulling than being stuck in a cell to stew on your thoughts.
Sometimes you have to remember that it not every step is a fun one. Whenever my characters have been in cells, I just hop to alts, play other games, or hop on when there's action. It's not that big of a deal.
Edit: Just to be clear, I am a fan of prisoners being near eachother, and requirements to put them close to eachother in the Citadel, which has blocks of sometimes four to six cells in LoS of eachother. In the Gendarmerie's case, I've been asking for years for the cell doors to be changed to ones that do not block LoS, which the Gendarmerie only ever has 2-3 prisoners at a time. Put them side-by-side, issue solved.