Generic pedo-transfer functions to compute field capacity and wilting point from sand/silt/clay?
As far as I know, there’s no one definitive pedo-transfer function (PTF) but there have been several studies that train functions against multi-site databases, rather than just deriving site-specific functions.  These PTFs typically define soil moisture-conductivity-suction relationships based on silt, sand and clay fractions from which you can read off field capacity (suction = -10 kPa) and wilting point (suction = -1500 kPa).  Which PTF you use will depend on the soil moisture-conductivity-suction hydraulic description that your model uses.

For Clapp and Hornberger hydraulics there's the Cosby et al (1984) PTF.  This is trained on data from the US.  It's quite possible that there are more recent PTFs trained on more and better data, but this one is still in use (e.g., by the UK Met Office).

For van Genuchten hydraulics there's the Wosten et al (1999) PTF (see also Wosten et al, 2001).  This is trained on the European HYPRES database, and you need organic percentage and bulk density in addition to the silt/sand/clay percentages.  There’s also ROSETTA (Schaap et al, 2001), which uses several PTFs hierarchically and is trained on data from North America and Europe.

Note that these hydraulic properties are difficult to measure in situ and in the lab, so the fitted PTFs often explain quite a low fraction of the variance in the training data.  You might be able to use this information to inform your priors, which should probably be quite conservative.