diff --git a/hgext/rebase.py b/hgext/rebase.py
--- a/hgext/rebase.py
+++ b/hgext/rebase.py
@@ -799,6 +799,19 @@
         elif repo.currenttransaction() is not None:
             whynotimm = "rebase run inside a transaction"
 
+        # in-memory rebase can be configured not to run if a mergedriver is
+        # configured; this should be used only if the driver's preprocess()
+        # scripts might use the working copy.
+        #
+        # (Note: Even without this config, IMM will still abort if any files are
+        # marked by preprocess() to be driver-resolved; however, sometimes the
+        # preprocess() function itself can't be trusted.)
+        #
+        # TODO(phillco): Replace with decorators on the scripts themselves
+        elif (ui.config('experimental', 'mergedriver') and
+              ui.configbool('rebase', 'experimental.inmemory.nomergedriver')):
+            whynotimm = 'mergedriver enabled'
+
         if whynotimm:
             ui.log("rebase", "disabling IMM because: %s" % whynotimm,
                 why_not_imm=whynotimm)