If you really want the convenience of using a measuring spout, you can just dump the measured powder into something else to transfer it to the barrel. This is still somewhat quicker and less fussy than carefully pouring into a measure. As I believe someone has already mentioned, you could make/find something that is a snug slip-fit over the measure/spout and closes off the end, and attach it by a lanyard to the horn/flask. Upend horn/flask, open valve, shake, close valve, pull horn/flask up out of transfer thingie, pour powder into barrel, replace thingie. This thing would also serve to keep water and debris out of the measuring spout.
For what it's worth, my impression is that measuring flasks were not common until mid 19th century, and earlier they were more of an middling-sort or upper-class thing, and sometimes military. After they did become common, there were MANY admonitions in books and articles against loading directly "from the flask" for fear of possible explosion. I've read period accounts of these accidents, often with a mangled hand resulting. The alternative urged was to load "from the hand" or "from the palm", using the palm of the off hand as the intermediate transfer vessel, with a minor burn as the worst result of flash. This procedure is easy to do with a shotgun with a round barrel little larger than the bore, but harder with a rifle.
Regards,
Joel