Looking at the French conditions from the perspective of Latin it seems to me that what is called in the French context the present unreal or 2nd conditional (si + impf., pres. conditionnel) il serait très content si vous lui écriviez "he would be very happy if you wrote to him" corresponds to the Latin future-less-vivid or should/would condition, i.e., it is essentially prospective: "he would be very happy should you write him". But what if you want an unreal condition that is truly in the (progressive) present, the kind that you can make logical inferences to the real-world situation from, e.g., "if it were raining the grass would be getting wet" (the grass is not getting wet, therefore . . . )? Does French have a way to express that?
I would still use "conditionnel + imparfait".
If someone tells me it's raining and I see the grass is not wet nor is it getting wet, I would reply
S'il pleuvait, le gazon serait mouillé (le gazon n'est pas mouillé, donc il ne pleut pas)
La subordonnée n'est réalisée que dans le cas où les conditions citées précédemment sont remplies et à partir de là, on peut en déduire qu'il ne pleut pas.
Using the progressive for mouiller here doesn't sound natural to me.
It requires no change in tenses to express that it's happening right now. It's just a matter of adding "en train de". It's just that it's very unsual to write mouiller in the progressive form (le gazon serait en train d'être mouillé (par la pluie)?).
To express a rejected supposition in the continuing present, in French you use the "imparfait de l'indicatif".
- S'il pleuvait, l'herbe commencerait à être mouillée.
A second option, which provides a more specific representation of the situation in the light of the fact that in your sentence the verb "to rain" is meant to say "begin to rain" rather than simply "to rain", is preferable, I think; the grass is getting wet only at the begining of the rain and then, rapidly, it's just wet. So, instead of translating by "pleuvoir" we use "commencer à pleuvoir".
- S'il avait commencé à pleuvoir, l'herbe commencerait à être mouillée.
The following turn which consists in using the "imparfait du conditionnel" with an inversion is literary and not to be confused with the other two: it expresses a possibility, not a rejected supposition.
- Pleuvrait-il, l'herbe commencerait à être mouillée.