I understand. In this case, may be you should take a look at NME (the node material editor).
You could likely fairly easily create your shader with low impact on performance.
There are already a number of base examples available you could may be use as a start. Unfortunately, they are all a bit dispatched here and there. I know there’s a project on with V6 to consolidate all NME examples. Meanwhile, you would just need to make a search in this forum and come back here with your questions.
The team and community has some incredibly knowledgeable and willing to help individuals you can cc to help you with creating this shader that would match your needs.
Edit: Oh, And I forgot this GLSL editor that just came in lately. May be this could also help.