FriedChicken : I simply replaced my PPD and filter files by your ones, which were detected and used by cupsys and I added a printer in cupsys configured with that driver, I changed the paper type and I applied my changes, and nothing changed in /usr/Brother/inf/brDCP110Crc . I just tried again everything I just described with DEBUG set to 1 instead of 0 and I did not get any outputs in the /tmp folder. I can't know if changing paper type worked because I don't have a Brother printer.. In your case, you are able to switch between A4 and Letter using cupsys and it works perfectly? Is /usr/Brother/inf/brDCP110Crc updated with your changes? If yes, there is probably something important that you modified in your side that we don't have.
Till : DCP100C does not use shared files in -common, you can refer your the Brother packaging wiki page I made for this since it's really complicate. The only thing inside the -common package is /usr/Brother/Printer/brcupsconfpt1 needed by bh7 and extra, it's not a script but a binary which we compile from source of bh7. -extra actually contains all printer drivers that did not have any source tarballs, and a good part of these should have been included in bh7 source package by Brother because they use exactly the same files. I keep them in -extra because they are orphan and use different license. To avoid collision, I keep /usr/Brother/Printer/brcupsconfpt1 in -common.
Since I don't have any source code for the printer drivers in -extra, I have no way to really verify the differences between the upstream binaries and our compiled binaries but Brother never did two different cups wrapper with the same name. Anyway, it should not have any impact on DCP110C which does not use this shared cups wrapper.
When comparing all upstream binary brcupsconfpt1 in bh7 with diff, they are all identical.
When comparing all upstream binary brcupsconfpt1 in extra with diff, all packages that have the same version number have identical files.
When comparing all compiled upstream brcupsconfpt1 in extra with those in bh7, all packages that have the same version number have identical files.