How many LUNs are we talking about?
Even though you are expanding the journal LUNs and allocating more space to image access that doesn't mean that you will not run into space issues. 20 days is a long time to keep RP in image access mode.
What about running SRM Test and then cloning the LUNs at the DR site? Once the clone completes abort the test and your production stays as is. Present the clone LUNs to the DR host and perform any tests you want. You won't be able to replicate the changes from DR to PROD but it doesn't sound like you want that anyways.