A quest with a combat dummy is on my list. I have the scripts from Daggerford, but I have a hundred other things to do
Though, I think something simpler like having the leaders of HH and RM give a letter of recommendation should be enough if PC's have completed the quests.
I will look at the Argent Legion quests and see if I can update the Marhsal convo and Argent Legion Post sergeant scripts for recruitment if you have completed all the RM quests.
There is a militia badge and I can add a store to the armor in Rauvinwatch Keep.
BTW, for DM's, the militia badge is in the toolset under badges if you want to do this manually with the Post Sergeant in Rivermoot. Along with armor (you can probably do a trade for trade for Argent Legion armor, ie regulat chain for Argent Legion chain, regulat shiled for Argent Legion shield)
Each post sergeant has a contigent of Argent Legion, but all militias fall under the Argent Legion, so the Rivermoot Guards are actually part of the Argent Legion (I am updating their unfirms if it has not already been loaded by Indio already).
Rivermoot should have a militia unit commander if it is not the marshall that falls under the umbrella of the Argent Legion.
If Seth or anyone else has completed all the quests and wants to join the milita, I would say just visit the marshall, get a letter of recommendation, and then visit the post sergeant (who doubles as recruiting sergeant for small towns).