How about making a sticky that's updated once/twice a month at beginning and/or end, that has "X" tasks completed out of "X". And in big bold letters above the task amounts, a warning/notification that states "These numbers WILL change based on various factors." Then list a few factors like "bugs that pop up, different ideas being added, etc" this way people get a very generic idea of how far along you guys are, without an actual release date. They get to see that tasks are being completed so they dont feel like the project is at a dead standstill and when the total task amount goes up, they can still rest knowing things are being added and/or fixed.