I recently exchanged posts with somebody about the HKS 941 (sorry, can't remember who) and he said it worked great with his P99 even though the P99 wasn't specifically listed as an approved handgun for the 941.
I've been trying to find a Canadian distributor for HKS so I can get one (it would work for my Beretta Brigadier too) but no success so far.
It works very well. Use the second slot. Don't be put off by the fact that the P-88 is listed, but the P-99 is not in some advertising or packaging. HKS has not revised all of its advertising and there are still some 941 packages out there that do not list the P-99.
On all 4 of my fullcap mags (Mecgar mags, from the factory) the baseplate didn't fit in the grooves on the HKS speedloader. It did fit perfectly on the 3 Clinton mags though (also Mecger, also factory.)
So I dremmeled off the top rail, so that the base of the mag just sits against the second rail. Don't look pretty, but works great.