I made a gruit just as you're describing, except gluten-free, with rice syrup and brown sugar instead of malt. I used flaked oats and malted quinoa as steeping grains IIRC, steeped in a muslin bag. Same herbs and everything, except I did 1 oz each in the boil and 1 oz each dry-herbing in secondary after boiling them. It was only my third batch of gluten-free beer ever, but it came out really strong (10-11%, I think...I didn't really intend that, I just didn't know how to calculate gravity at that point and kinda just guessed) and kinda medicinal tasting. Not bad, just weird! Might probably taste a lot better not gluten-free, but the herb combination sure is strange. I'd say try making a small tea first and trying that, before you go and brew it. My feeling is it make a great winter seasonal. Definitely wouldn't play well with hops.
So yeah, totally add some steeping grains.