I think Machiavel was exaggerating, but maxing out every nobles unit's size with the best units available would be something all that gold could do. It could also be used to make recruitment centres, build up fortifications, place militia. Ten thousand gold could be used pretty quickly.
I was not exagerating. D'Hara has massively funded our allies in the past, and we are but a single realm. Morek and Astrum, alone, should be able to give at least thousands weekly, near the 10,000 mark at least. But with all of them? With Iashalur, Corsanctum, Farronite Republic, and Libero (which basically did nothing but could have easilly been bullied to do something), I think that 20,000 gold a week is very ambitious, but not impossible.
Terran has too few nobles? There was a time where Barca and D'Hara were also in the war. No need to dump it on Terran alone. They also could have incited nobles to pack up and go to Barca. But how many rulers encouraged their nobles to leave?
So considering this, if some people, be them the rulers of the astrocracies (doubtful) or the church elders (more likely) want to turn Mendicant into a rallying figure to oppose... Well, good luck. You could turn him into a devil figure you never actually act against, perhaps, but for some reason I doubt this is what's desired. If you wanted to use a war against him to get everyone working together, I just don't see that happening... You are too far yourselves, and those who aren't won't sacrifice their realms for people who don't really care to win the war anyways.