You are right, Forest uses forespack.ini to know check if the texture dialog path must be shown. Instead editing this file, we suggested you an alternative way (uninstall / reinstall), but we just realized this procedure doesn't work in all cases (specifically, only when upgrading to a newer Forest version). I apologize by that.
Regarding the .mxp files, usually we suggest to edit the 3dsmax.ini directly because in this way you have more control over the texture paths. Also, in the render nodes Forest adds their own local paths to the configuration, but they would be deleted if a .mxp file overwrites the .ini, causing that Forest textures are not found.
On the other hand, in Backburner it's possible to assign a .mxp file when submit a rendering job, but we are not completely sure what is the behaviour of the DR nodes regarding the .mxp files. Please, let us to do some tests and ask to ChaosGroup about it. We'll post here as soon as we more information.