Ok, the script is now available. I haven’t had a chance to test thoroughly in Gimp 2.8, but it seems to work ok. So, as of the moment there is only one version of it (1.0) for both – Gimp 2.6 and 2.8. For the formulae – all standard python Math operators and constants are supported plus phi and phi_inv.
Grab it in the Downloads section.
If you have questions or feedback, drop them in comments below.