One thing I would like to be able to do is to tell Picard to pad disc numbers with leading zeroes so that the disc number tag in all files of a release have the same number of digits. So if there are 110 discs, disc number tags will be 001 … 054 … 110.
I haven’t found a setting to do this. I presume it can be done with scripting, but I am not adept at scripting. If this isn’t currently possible, please take it as a feature request for 2.7.
Okay, when I said I wasn’t adept at scripting, I meant I know nothing about scripting. 
Be aware that for MP4 tags disc and track numbers are stored as integer values. Hence the padding won’t have any effect there.
For this reason and others, this might be a better feature for a music manager than a tagger. MusicBee, for instance, has a feature that will standardize the way track and disc numbers are displayed. Also, in either Picard or MusicBee, you can use a script to apply padding in file or folder naming.