…in fact yes that’s all it takes (this is the R6 CR3 sample from raw.pixls.us):
cd ext/rawspeed
git remote add cytrinox https://github.com/cytrinox/rawspeed/tree/canon_cr3
git fetch --all
git checkout canon_cr3
cd ../../
rm -rf built/ext
cd bin
make -j20
…and it’s even slower than the CR2 decompressor was, it seems 
i also have this local diff
diff --git a/src/db/db.h b/src/db/db.h
index 665d924..b5311e6 100644
--- a/src/db/db.h
+++ b/src/db/db.h
@@ -118,6 +118,7 @@ dt_db_accept_filename(
const char *f2 = f + strlen(f);
while(f2 > f && *f2 != '.') f2--;
return !strcasecmp(f2, ".cr2") ||
+ !strcasecmp(f2, ".cr3") ||
!strcasecmp(f2, ".crw") ||
!strcasecmp(f2, ".nef") ||
!strcasecmp(f2, ".raw") ||
(and no i’m not proud about it) will push at some point, but need to run now.