With regards to the medium- to long-term solutions, one of the options that we discussed at the Openmod workshop in Zurich was the application for a COST networking grant which could among others allow us to organize larger conferences, that are currently beyond the budget of individual inviting institutions.
This will not help in the short term, but could at least provide some relief in the medium term, until participant numbers outgrow the increased conference sizes that could be enabled through the grant. Once that happens, we may need to think about parallel / regional / topical sub-events… But I guess that is a bridge we should cross when we get there?