So, looking at that Long List of people that could be asked to contribute...
[ ] Averland's army is currently mobilized to mop up the last vestiges of a defeated Waagh; perhaps they can be convinced to contribute.
[ ] Hochland's famous snipers could come in useful in a prolonged siege.
[ ] The forests of Mordheim will keep. Invite Ostermark to bring their armies into Sylvania, so that they might actually achieve something for a change.
[ ] Anton is already in Reikland seeking assistance from the Emperor, but perhaps a greater focus here could pay dividends.
[ ] Talabecland's knights appear to be having a grand time, perhaps they could convince more of their fellows to join in.
[ ] More cannon. More cannon. Send to Wissenland.
[ ] There's one neighbour that hasn't been asked for help. Maybe the Moot could contribute somehow?
[ ] Send for Battle Wizards from a College of Magic (pick which, can be taken multiple times).
[ ] What purpose could be more holy than this? Request aid from the Grand Theogonist.
[ ] Surely Van Hal has a lifetime of contacts and favours to draw on. Bring the Witch Hunters in.
[ ] The Cult of Ulric are no friends to the undead. Call them to battle.
[ ] The Fellowship of the Shroud are technically Tilean, but their enmity of vampires is unquestionable. Extend an invitation.
[ ] The Throng of Zhufbar is already seeking reinforcements, perhaps the backing of an official request from Stirland could help.
[ ] The Slayer Keep, Karak Kadrin, is close enough to Sylvania to inspire a dislike for it. Ask if they'd be willing to join in.
[ ] The dwarvern hold of Karak Angazhar in the Black Fire Pass is small compared to Zhufbar or Karak Kadrin, but they've closer ties with man and a greater interest in trade; perhaps they'd be easier to convince to join in.
[ ] Apparently there's dwarves gathering for some sort of revanchist crusade into the Badlands. Perhaps they could hone their edge against this fortress.
[ ] Drakenhof is very close to one of the utmost reaches of the Stir; perhaps the riverine forces of the Empire could contribute to the fight.
[ ] Sylvania is technically part of Stirland, so this could be considered defensive. Raise the militia. (pick which segment of the militia, can be taken multiple times, check the Military of Stirland threadmark for militia details)
[ ] Seeking mercenaries has, to put it mildly, gone rather well so far. Seek out more.
[ ] It seems Countess Gabriella is content to sit back and watch events unfold. If your suspicions are correct, and you don't mind dabbling in what might be considered heresy, her personal intervention could be a great help.
I'm thinking there are three distinct strategic criteria to weigh them on when planning this, if one wants to be thorough about it.
1. How much help would they be?
For example, a Battle Wizard laying down fireballs on hordes of undead is probably superior to the average level of mercenary company we'd be likely to get.
2. How willing are they to listen to us?
Imperials are probably more willing to listen than foreign humans, who in turn are more willing to listen than dwarves.
3. How interested would they be in assisting?
Slayer Dwarves looking for a glorious death would probably be some of the most interested in attacking Castle Drakenhof.
And on each of these questions we're considering the
marginal improvement from Mathilde's extra letter-writing, not just base amount, so there's probably (for example) less value in writing to Reikland where Anton is already.
So, cutting various groups out of consideration where I feel they do too badly on one of the three questions.
IMO, Q1 cuts out mercenaries and the militia among others.
Q2 probably cuts out Ostermark ("leave your fight and join our fight!" might go over badly) and the Badlands Dwarves (I figure they'd be all "who dis?")
Q3 probably cuts out the riverine forces among others.
Then the rest can be weighed on overall score of these - Battle Wizards, for instance, are obviously popular because they score something like High/High/Average.