you can already. Drag the BMP ao. timesig counters into automation envelopes. The settings will change acording to the envelope shape. Impressingly, even the songeditor-design change in acordance!
BR.
Make sure to search to see if your idea has been posted before! Check our issue tracker as well, just to make sure you are not posting a duplicate: https://github.com/LMMS/lmms/issues