Funny, I've been shooting black powder since the mid 1970's and I've never had any issue with taking the 3 little screws out of a flask head to fill it. Never lost a screw, never had a problem putting the flask head back on and re-installing the screws. After reading some of these posts I must be gifted LOL. Using some type of funnel to fill an assembled flask certainly works, much like filling a powder horn, but from my very first one I've simply taken the flask head off to fill a metallic flask. It just seemed so much easier than fiddling with a funnel and trying to guess how full the flask was. To each his own I guess.
PS. I do own a couple antique flasks that the heads cannot be easily removed (screws had the heads filed flat apparently) and if I were to use them I would use a funnel just like I use on a horn. I don't use them though, they just sit in a cabinet with other antiques. I also have a couple antique flasks with the screw off heads like the one that a poster showed, they are nice and would certainly be easy to fill if I wished to get them out and use them.