Kalman Filter - Position/Velocity from Accelerometer and periodic position measurements
I'm not sure how well it works, but one way to tweak the model is to set the measurement noise variance for the missing position measurement to a large value (meaning high uncertainty of the measurement), just for that missing time instant.

Setting it to be "infinite" should have the same effect as what you're doing now: just equate a priori and a posteriori estimates.

What setting the variance to some smaller value might give is a way to keep "momentum" in the position change.

I've not tried this approach, so please take with a grain of salt!