Why not request each md5 file with wget for every *.tcz in your optional directory, then compare the md5sums to determine if your currently installed extension is an old version. I used this method in "tce-md5check.tcz" which fetches the most current updates in favor of the traditional database method.
besides, you only need to download the md5sums for extensions within your optional directory, not the entire repo md5sums.
You could also call tce-update, also used by APPS gui (both use the database method)
Note: the database method may not be aware of the most current updates as the database file can take a day to reflect recent updates. tce-md5check method will install the very latest updates which are often missed using the database method, this fact was discovered during testing of many other tce-md5check features.
Note: that with both tce-update and tce-md5check removing any md5.txt file from your tce store will prevent an unintentional update, thereby saving any modified extensions from loss. tce-md5check will also keep a local blacklist of extensions you might choose to blacklist from update or will download any missing md5.txt files for those you choose to allow updates.