One down, one to go
Update on the Brother MFC-420CN situation:
The Perl CUPS wrapper is done, and seems to be working fairly well, at least for just simple printing using only the default options.
On the SANE side, things are less than ideal. Despite finally being able to cleanly compile the brother and brother2 back-ends against the latest sane-1.0.18 sources, there are problems with required functions not being properly defined, and hence unusable.
For those that care, the Brother binary SANE drivers don’t work because sane-1.0.17 did not have a workaround that allows multiple processes to access the same USB device simultaneously. This is required for multi-function printers, where the usblp will reserve device at boot time, causing SANE to always receive a BUSY status when querying for the scanner.