Deny the SID deviation until the plane has reached the MVA (causing the aircraft to enter the storm cell), or
Squawkstandby, as a pilot (A320 PIC), I disagree with this assessment; it will not likely cause the aircraft to enter the cell.
I understand your dilemma; if your regulations do not allow a deviation from SID/ATS route below MVA, you don't want to use the word "approved" as you cannot actually legally approve. Also I agree that, however you view this situation, some pilots will take this as a verification that the deviation is safe and you do not want to introduce that doubt.
But if you deny the deviation, it is unlikely the aircraft will enter the storm cell. The commander (if he/she has *any* sense whatsoever) will use his emergency authority to make the deviation anyway.
How about the wording "Unable to approve below FLxxx/xxxxFT due minimum vectoring/safe altitude".
Any pilot with some sense will either: expedite until reaching safe, or turn anyway reporting unable to follow routeing. Probably a combination of both.
Either way, you have made it clear that a) you do not take any responsibility for the aircraft course, and b) there is no traffic issue as you specifically mention terrain.
Let the commander do the rest. He did, as already mentioned, take-off into known storm conditions so let him work out his plan without putting it on your shoulders.