The exact forecast of the snow amount, in term of losses and periods, is quite impossible. There are some possibilities linked with meteo data which indicate also snow coverage, but as Mr. Mermoud has written, there are several different factor which influence the snow coverage effect: snow type, ambient temperature before, during and after snowing, module tilt angle, wind, humidity, irradiation, etc.
I think that the evaluation of the expected losses due to snow coverage is responsibility of the user, as well as for normal soiling (also difficult to appreciate as the snow).
For snow losses, probably could be a good approximation to define some days per month with the expected snow coverage of the month (a sort of unavailability of the system) and concentrate the expected monthly snow losses in the selected days of unavailability, without taking into account the partial covering effect. The hourly or daily behavior would be probably not realistic but the monthly values could be good enough to evaluate possible influences on the design of the pv-plant (narrow row distance, higher tilt angle, etc.).
However I think that the snow losses, as well as the soiling losses, should be taken into account by PVsyst before the other losses (Horizon, Shading and IAM losses).