Nope, you'll have to change the scripts for that purpose. Open /usr/local/lib/7dtd/commands/updateengine.sh, go to line 77 and add "-beta alpha10.3" right behind the "294420":
./steamcmd.sh +login $STEAM_USER $STEAM_PASS +force_install_dir $SDTD_BASE/engine "+app_update 294420 -beta alpha10.3" validate +quit
Should work that way. You'll have to do a update with --force then though as the buildid will obviously be lower than the current one.