It shouldn't be a problem, but it's always good to have a few woody shrubs like Viburnum (Arrow wood) around for the larvae of Hummingbird Clearwing Moths as well as supports for Monarch chrysalids.

I agreed with Leafhead, having a few woody shrubs like Viburnum is a good idea..

My challenge here is that I want to keep everything in this bed under 3 feet. It gets pretty hot surrounded by pavement, and is prone to drought at times. I won't put anything there that's going to need supplemental water.

I'm wondering about Amorpha canescens, though perhaps that's not woody enough. Rhus aromatica 'Grow Low'?

If you don't want any woody shrubs now, but want something more substantial quickly for chrysalids try adding some woody branches. I often take sturdy dead branches and twigs and trim them to a good size with lots of horizontal branching. Then I push them well into the ground in amongst the plants. They act as plant supports, bird and dragonfly perches, and maybe even chrysalis hanging spots.

Great idea, thanks!. Small branches fall out of that oak all the time -- especially this time of year, they could be easily hidden among the grasses.