We have encountered this a number of times in glasshouse designs and often there are different targets from the botanist, sometimes it is hours over a certain brightness, as could be calculated out of an annual illuminance simulation, or as a fraction of outdoor light availability, which may be as simple as a daylight factor calculation. If the botanist is happy with illuminance as a proxy for PAR, that can be worthwhile for designing around performance of low-light spaces that may not receive direct sun.
We generally let the Botanist calculate PAR via a preferred conversion method from that illuminance value (perhaps it is from local measurements), though I appreciate this does not take into account material reflectance of the non-visible PAR wavelengths (keeping it simple).
The language in agribusiness and plant conservatories is generally a daily light integral, which is simply the cumulative illuminance over a day, measured in lux-hours. This at least is a simple bit of post-processing out of an hourly simulation and can be built into a grasshopper workflow without much hassle.