I mentioned this to Jon at Summit and finally got around to testing it out. I haven't tested this in production so I can't vouch for the performance hit if there is any. Shouldn’t be…
Here’s how it works, you would have a customization layer that would be named nested-tables.xsl that has an import statement to the renamed original, say nested-tables-orig.xsl. The only things in your customization layer would be the xsl <template> tags that you need to modify and the import statement (and some other minor stuff see example). Whatever is in the customization layer would over-ride the originals with the same name *and* attributes. Unfortunately you would need to copy over the entire <template> block, even if you're just changing a small part of it.
So hopefully when a luminis patch blows away your nested-tables customizations you just rename the Sungaurd version to nested-tables-orig.xsl and copy back over your customization layer. If anyone knows where the path to the nested-tables is set, you might not even have to do this step and just change the path to point your customization layer, which imports the original.
Note if Sungaurd actually made any changes to the nested-tables.xsl in the specific templates that you override, you would need to then merge the changes into your customization layer, but I don’t imagine a whole lot of that goes on, and it would be a lot easier to diff the changes from the previous Luminis release to the new release than the new Luminis release to your customized nested-tables.xsl.
My plan now is to try changing image filenames, and possibly using a stylesheet override so I’m not making near as many code mods.
I’ve attached a sample that adds [!!] before each channel.