I've discussed this with the tech team. It is not such a good idea to allow a rescan from a specific block, because of various parts of the wallet state that must be synchronized properly.
Again, we would like to understand the problem you are seeing and its causes, since a rescan should never be necessary with normal usage.
In the meantime one suggested workaround, if the issue is with a specific stream, is to unsubscribe from that stream and then resubscribe to it. This should be faster than a full rescan of everything in the blockchain.