Is it possible to use F5 as proxy/reverse proxy server?

You are very close. Try this:

 
 set group ::datagroup 
 log [subst $$group] 
 log [lindex [subst $$group] end] 
 

You had an extra $ in the variable of the datagroup.

I would also suggest you simply modify your host_map to contain the :: for the group name so you don't have to add the extra variable set command.