Void-only and Skyblock type playthroughs are functionally locked out of infinity dust. Tier four MicroMiner requires it for the reactant dynamo and thus makes it impossible to get. This makes the playthrough impossible to finish, as you would never be able to make a large portion of the TE dynamos, and thus you would never be able to make the Ultimate Generator.

Adding a recipe either from Omnicoins or from something that makes sense like using durability on a flint and steel and consuming an obsidian block.

The S:infiniteBlocks option in enderio.cfg allows you to add custom blocks that can produce Infinity Dust when lit, and I:enableInDimensions (or B:enableInAllDimensions) lets you enable it in Void world as well.
On my server I'm using compressed obsidian (armorplus:compressed_obsidian) exactly for Void-only playthroughs, works super well and feels close to original (also because getting obsidian in void world at LV stage is a fun project of its own). Would that work? Can send a PR if so.

I don't really want to implement a new mechanism for obtaining grains at this time. There are different resource generation methods required for a purist Skyblock experience that have implications on progression that also affect those who do not choose to play exclusively in a void world.

Omnifactory is not a pure Skyblock/Void pack: you are given the option of building in a void world, but we do intend for you to do some things in the Overworld, Nether, and End. Damien chose to stop trying to support pure Skyblock around 1.1.0, when the life motes system was removed.

If you find it more enjoyable to adjust configurations for your server, you can always do so.