I just posted about mobi2mobi here
https://www.mobileread.com/forums/showthread.php?t=17718
and that Perl program can change the title but I am not sure if it works on DRM:ed files since I do not have any to test with. If the title data is the last data in the header than it probably will work.
You can edit the file in an editor to change the title if you also change the 4 bytes that specifies the length of the title.