You know approximately when the AGM will be held. The EC might not be being secretive, they might be still working out who of the EC has which days available, when the managing agent can attend as well and when the usual venue can be booked. That aside, why not just write to the EC with your motion to be placed on the agenda of the next general meeting, whenever that may be? Copy your letter to the managing agent and anyone else you care to if you are concerned that they might ignore it.
If your proposal requires background information or detailed explanation you could provide a page or two to be mailed out with the meeting notice whenever that occurs, presumably in the next few weeks if you are getting close to the AGM anniversary. The EC cannot refuse to list any motion that you put. Having said that, if the EC does not support your proposal they could put a motion of their own in opposition to yours or include their own statement about why your proposal should not be supported. That would not be inappropriate since a function of the EC is to develop strategic matters for the OC. Once it is on the agenda your proposal can be debated and it can’t be left off the agenda.