Lorenzo Meneghetti wrote in RE: Custom Fields - how to style URLs?:
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If you are interested and if you have the ability to edit redmine code, a few years ago I wrote a new method that allows you to do exactly what you ask. It seemed like a missing feature. In the end I didn't publish the patch because I had no time to write the test method too. :-)
If you are interested I'll share it here.
At least, it would be cool to see the solution. This could be nice feature.