Entities inside a group which do not have an explicit material will inherit the group's material, so it already does work this way, no?
Mate it does work like that yes! However only if the geometry has NOT been mapped! With these brick materials like a lot of arch materials map placement is critical and cant rely on a simple paint to default!
Other than that, I did not really grasp the exact meaning of your idea. I think you are basically saying that rather than MXMs being assigned as a function of their association with an SketchUp material, you would like that they could also be assigned on their own to groups, separate from the idea of SketchUp materials.
So, an MXM file path would be attached directly to a group's data, I suppose by right-click menu or similar. Is that even close?
Mate you got it spot on in your assumption! Items could be mapped with a simple scaling material to set critical map placement, in the example a simple B/W map of the brick / mortar pattern ie: All brickwork mapped with the same SU material. Geometry that may be assigned differing variances of brick could be grouped and via yes right click or via the "Assign to selected entities" button on the material section of the scene manager! However this later has an issue that an MXM can't be loaded then assigned presently without the selection of an SU material!
As mentioned earlier as maps from suppliers, those made by the users, or in collections tend to be of matching size for particular materials and this method of assignment would greatly leverage this aspect.
Again the current reliance on SU materials always defining MXM assignment requires any critical mapping to be redone to alter assignment other than global. I've got a great workaround to shorten this process though still can be a drag! As you can see above my model I have flying planes of my brick materials, these panels are set all from the same origin and set to projected, this than allows me to select a material from the corresponding flying plane / axis and paint to all selected same facing geometry and then sample and paint from the other plane / axis and paint to the others. For this type of building construction this is a very fast and effectual simple way to model / paint / map as the brickwork is the standard unit for sizing the exterior walls and the geometry can be push/pulled out using the map as the measurement reference, but still all this editing needs to be done manually for each test and has limitations as all the brick sills need to be replaced / edited with a new component
On another matter regarding material assignment, at present the plugin does not display the name of the linked MXM except for the tool tip which is ok until one wants to change the link, when browsing for a new material the current MXM name isn't displayed in the browser instead "choose file" then if one has to pick an alternative yet similarly named file no reference to the existing name is available and if one forgets to check via tooltip one has to shut out and reopen browse to allow for a quick confirm. Would be cool if either the current material name is either 1. displayed in the scene managed so to be visable whilst browsing or 2. the existing name appears in the browser inlieu of "choose file"!