I think you can alias it through autoexec.cfg, found in RealRtCW\main. Try the following and see if it helps - change 'F5' in the last line to your liking:

set tp1 "cg_thirdPerson "1"; set tp vstr tp2"
set tp2 "cg_thirdPerson "0"; set tp vstr tp1"
set tp "vstr tp1"
bind "F5" "vstr tp"