is a man of many hats -- and heading into the weekend, he put on his wings and picked up a wand to play a musical cupid for none other than Matt Iseman.

Check out this video, obtained by TMZ, showing the former Mr. Olympia doing his best conductor work Friday out in Austria ... where he's there for a mix of business and pleasure. As you can see, he was leading a band and singing a tune, with a lot of people around.

We're told this was actually at a pre-party ahead of the Hahnenkamm Races -- a major skiing event out there that they do annually ... and which draws big crowds across the world.

Anyway, while Arnie was doing his thing with the band ... Mr. Iseman -- famous for hosting "American Ninja Warrior," not to mention winning the "New Celebrity Apprentice" with Arnold in 2017 -- was also there enjoying the fun ... but then, things turned romantic with his GF.

Our sources say Matt accompanies Arnold for pretty much all his appearances -- remember, he was just hosting this

for that troublesome watch -- and he MCs those too.

That's probably why Arnold helped him propose to criminal defense attorney Britton All ... who was clearly not expecting to get engaged right then and there. Arnold was obviously in on it. Oh, BTW ... John Kerry was in the mix there too -- like we said, a who-who type shindig.

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Safe to say ... Britton said yes -- and Arnold was there to lend an assist ... congrats!!!