Rather than (I assume master) craft ambassadors, I always got the impression that the masters stayed in one place, guru style, and the jorneymen/apprentices came to them. So you can send out your guy with a backpack full of food, booze and a *Golden Trumpet* to pay for tutorship and lodging. This teaching would be much more linear than training. For example you could gain one level on each trip. When training is more balanced, this would be a very big deal when you're in the master range. To counteract this, receiving training from a master should be very, very expensive if you're good since you need an even better teacher. A trip would take a year or so. Leave in spring, come back next year. Or not at all, if he's killed by carp. Or come home in the summer after being robbed. Or run of with a circus and come back as a Legendary Mimer.
Only Masters and above were allowed to join and teach in the historical guilds
Didn't real guilds also forbid non-guild practitioners? I can imagine requiring a Master level craftsman to allow selling stuff to caravans, especially foreign ones. This would of course be dependent on which kinds of guilds there are, how strict they are, . For example a dwarven civ might not be so concerned about shitty kitten biscuits being sold to the mountainhome, but woe to the fortress that exports dwarven metalwork abroad (ally or not, reputation is at stake!) without guild approval.
The global civ guild or a local guild chief/council could also set (primarily lower) limits on the quality of exported goods.
Of course care should be taken to not turn such mandates into a you-lose-because-the-computer-says-so situation. But I think this can be avoided by tolerating lower level exports to the mountainhome and restricting it to just a few areas depending on each civ. Trade your shitty stuff to the mountainhome for cash (when that get's tradable) and use that money to get elven wood and Giant Cougars.
It's still legal to produce whatever you want for your own use. And even if it weren't, you either don't have a guild representative and they have no insight into your fortress, or you have and your production is therefore authorized.
The first craftsman in a field to reach a required level would be appointed local guild chief. After that they could be elected from viable masters, like OP said. Non-masters are considered working for the master as apprentice or journeymen depending on their skill.