I don’t know if you had any interest from others in your generous offer, but I would welcome any and all support implementing a hidden Markov model with linear predictors on cognitive test data we have collected over many years. Using brms would be icing on the cake. The aim of the project is to identify “improving” and “worsening” latent states from the test performance of (hundreds of) people, and relate these to personality and individual differences. Happy to discuss further if this still suits your interests…
Best,
Rich