There doesn't seem to be
any way to pass additional options to the rhnreg_ks command that the rhn_register Ansible module builds up.
The only two workarounds I can think off are:
a) Amend the /etc/sysconfig/rhn/up2date config file and specify the proxy details in there. See this
SO answer as there may be missing config lines for proxyPassword / proxyUser which you will need to add manually.
b) if you replace /usr/sbin/rhnreg_ks with a script which calls the original /usr/sbin/rhnreg_ks with the --proxy + --proxyUser + --proxyPassword options + whatever is passed on the command line.
Both of those are ugly since with a) you have credentials saved in a file and with b) your modifying sbin contents.