I can understand that you would not want to add a complete layer. It may make the soles last longer but there are downsides: reduced ventilation, moisture may get trapped (which may damage the sole and/or welt). More importantly even, the way the shoes wear may be affected because of the extra layer, small as it may be.
But if the toe tip is first to wear I can't understand why you wouldn't add steel tips, because none of the above downsides apply in case of good sunken rust-free steel tips. If you don't trust any cobbler in London with this work you could consider sending the shoes to Nils Kalf in Amsterdam, he is a very good craftsman as many users of the Dutch version of Styleforum will confirm. The charges for sending the shoes back and forth will be relatively hefty though.
I have rubber toe taps on my Vass and I prefer those to the metal version. They have the advantage that they still wear down through use, although at a slower rate than leather, hence to some degree matching the wear on the midsole.
But in the absence of toe taps on the JL, what's the worse that can happen if I wear down to the welt at the toe? I think the welt can be replaced during a recraft without having to replace the insole.