Ways to solve this could be potentially be
(1) to allow to fix parameters to certain values. This may be a feature implemented in brms 3 but I haven’t thought about it in too much detail yet (though should be possible although painful for me).
(2) allow deterministic formulas that is formulas that don’t include any coefficients at all (i.e., set them to 1; or 0 for the intercept) but otherwise behave like linear formulas. Not sure if this is a something reasonable in the grant scheme of things though.