Unfortunately no. At least not in the way you describe it.
There is an alternative, but it's little like what you really want. Most of the 3rd party rcon tools have a "reserved slot" option. This is not really a "reserved" slot, but will kick the player with the lowest score, and will not kick those that are set up as part of it's "protected" groups. The problem with this is that it requires someone to manually use the function. That means the command to open a slot has to be issued by someone manually, either from within the server, or on the rcon tool's CP.