Unfortunately, that's because the RefCloud program might simply use a global variable which caches the value from registry. I.e. it only reads such value once per session, and then saves it somewhere in RAM for future reference. Only when actively changing these values may it again reference the registry. I'm not sure which version of acad you're using, on my Vanilla 2012 it works fine, e.g.:
Command: REVCLOUD
Minimum arc length: 50.0000 Maximum arc length: 50.0000 Style: Normal
Specify start point or [Arc length/Object/Style] <Object>: *Cancel*
Command: (SetRevCloudMinMax 5. 5.)
T
Command: REVCLOUD
Minimum arc length: 5.0000 Maximum arc length: 5.0000 Style: Normal
Specify start point or [Arc length/Object/Style] <Object>: *Cancel*
The RevCloud command is implemented in ObjectARX (AcRevCloud.arx), thus it's near impossible to get to this thing from anywhere else.
All I can think of is to have your lisp issue the RevCloud command, then start it's Arc option, input the min & max, then issue a cancel (like in (command nil). E.g.
(command "_.REVCLOUD" "_Arc" m n
nil))