I'm working on a SF comic and would like to use grids to represent keyboards and such, or possibly create screen effects, like using a complex triangular grid on a computer screen.

Is there any way to use inkscape's grid feature to create a fill for an object?

can you be more specific about what it is you're trying to accomplish? I'm not really sure what you mean by "complex triangular grid".

You can certainly create grids and make them fills for objects. I can think of two ways to do this off the top of my head--clipping and object to pattern. First clipping.
1. Effects>Render>Grid whatever parameters you like.
2. Place your object over the grid, select both and do Object>Clip>Set. Done.
Next Object to pattern.
1. Effects>Render>Grid whatever parameters you like.
2. Select the grid, Object>Pattern>Object to Pattern.
3. Select the object you want filled. Object>Fill and Stroke>Fill>Pattern and select the new pattern, which should be your grid. Inkscape gives it a default alphanumeric name that I don't understand, but it should be the first one on the list.

That's as far as I can help based on what you've told us so far.

As to a complex triangular grid, an example is the inkscape background grid that isn't rectalgular.

I'd like to try hex grids as fills for objects to create a hex screen.

There's a grid render, but it doesn't seem to do hexes.

BTW, I tried creating a hex grid via tile cloning and using the merge paths to cut a rectangular hexgrid from it but SURPRISE! I couldn't turn the hexgrid into a path to use the path intersect function!

Select all your tiled clones and unlink tiled clones and convert to path. That should do it.