On the extremely rare times when it does turn out that my gate is still occupied (and this is just as likely, if not more so, when you're late as early), I just say words to the effect of "Well, we're here, 30 minutes early, sadly the bonus prize for being so early is that our parking stand is still occupied!"
Most of the time you can check out stand availability with the agent via company frequency, but by that time you're not far off top of drop anyway so holding would be the only option. The information so gathered can go out the window anyway as someone misses a slot or becomes unrestricted. In my experience, arriving significantly early is a function of it being a quiet time of the day (otherwise its departure slot or hold / lengthy vectors anyway), so stand availability isn't a problem.
The bottom line is I can't remember the last time I had to wait more than a minute or two for stand, but if we had had this policy in place earlier I would have been delaying departure, or holding / reducing cruise speed regularly.
This is a typical condition, namely that instead of being allowed to manage the flight in a sensible manner, I find myself constrained by some ill concieved edict from the company, or required to start making lengthy box 2 radio calls as entering the TMA.
Silly, Silly, Silly.
CPB