The windows rdp client does allow to connect to the 2 built in server rdp "console" sessions without requiring to have RDP "CAL"-licenses, which is especially relevant in Terminal Server situations.
The real problems arises when one tries to connect to a specific terminal server in an terminal server cluster, because in this case the connection is made to a random server, not the one specified.
This does not happen when the /admin switch in mstsc.exe is used.
Ive taken a look and freerdp has the ability to specify /admin too:
In FreeRDP\client\common\cmdline.c there is this block:
Unfortunately guacamole does currently not have the ability to pass this flag before it calls freerdp_context_new.
I think it could just be added here https://github.com/apache/guacamole-server/blob/master/src/protocols/rdp/rdp.c
which of course would require to add console_session to guac_rdp_settings which in turn requires persisting the setting and even worse, extending the java user interface which is just beyond by abilities ...