Incomplete Answer, because I have not tried it. This might set you in the right direction.
The problem is that the values of the LaTeX size macros (sizes and their associated baseline skips) are hard-coded in most classes (all classes?).
\normalsize would be the base size and the rest proportionally larger or smaller based on some fixed factor. This it not the case as seen in the files
or in the extended sizes
I think you'd need to create your own
size10.8.clo file that defines the following standard sizes.