Nope. It is allways a direct reference to the enum. To use multiple material classes, an interface would ne needed. That change would need to change every material reference, the most used class in GT. Also, every other mod using it would break. Again, that is how it is done in GT6.
Even if you freed up more than the 1000 IDs somehow, it wouldn't have been possible to write all that many new Materials. There is a 65536 Byte Limit in Java, meaning as soon as the Enum gets too large you cannot add more entries because the compiler refuses to do such a huge block of Code.
Also you said "Again, that is how it is done in GT6." Don't you mean GT5? or did you mean "GT6 broke everything in the Material regard and restarted"?