User defined session properties for custom commands

There are several patterns available (!U, !P) which can be used for custom commands. Is there any way to pass values of arbitrary properties defined in a session to the command?
I have a command which must behave differently depending on the session.

One way to implement this would be to add a properties section to the 'Advanced Site Settings' dialog. These properties can then be referenced in a custom command by a pattern like !'<property name>'.

No there's no such option. Cannot you use the hostname or session name in your script to decide on the parameters?

Using the host name is no option for me and using the session name is an option for simple cases only.
This might be a valuable extension to the custom command and would make them even more versatile than they already are.

I have added this request to the tracker:
Issue 1975 – User defined site properties for custom commands
You can vote for it there.