Re: @Gay Bear
Gawd, you don't half do it the hard way.

You'll be hard pushed to get any type of ball valve setup which isn't a tad heavy for the application. You could always use an isolation valve or similar, as they're not too bad on bulk and weight, but personally, I think you'd be literally introducing unreliability into the system, going with the scenario you have in mind.
You'd be better off just filling the system correctly, and leaving that inplace as a secondary measure if necessary, which is what it technically is.
Filling the system, (through the rad cap opening), slowly, is the key to success. If you use a small or narrow neck funnel, you physically can't fill the system too quickly, which is how most air is introduced into the coolant system. Fill the expansion tank to half full, (physically half full, not half between low and full), and then go for a quick run of around half to one mile, just to get the system upto temp, or thereabouts. It won't get hot enough to cause the expansion tank to overflow, but it will pressurise the system enough to draw any extra coolant necessary back into the system as it cools. Then, just monitor the levels for a couple of days, topping up via the rad cap and topping up the expansion tank if necessary. You shouldn't actually need to add any extra if you've done everything correctly. That last step is merely a precaution. Plus, any residual extra coolant in the expansion tank is fine. Anything above what is needed will just get dumped out of the overflow, if you've overly filled it.