Thanks for these tips David! I was indeed able to process my Sentinel-1 images to Force Level3 TSA, and calculate the spectral-temporal metrics. Here is my workflow in a nutshell:
1) I downloaded all Sentinel-1A +
1B IW Level-1 GRD products from Copernicus Open Access Hub, from my study area in both ascending and descending orbits, for the time period of two years.
2) I run my preprocessing script in SNAP to the L1 data, to create the level2 data. Output data is in WGS84 Lat/Lon Float32 Geotiff with ~10 m ground resolution (but still in decimal degrees).
3) I made a QGIS model to scale the L2 Float32 data to Int16, using scaling factor 10 000 and -9999 as nodata value (this could also be done with GDAL). The filesize dropped 50%. At this point, the output files were also named according to the naming convention you pointed out, e.g. 20170522_LEVEL2_S1BID_SIG.tif
4) Run force-cube to dice and resample the L2 data into my grid (the same I used for Sentinel-2 data). I used nearest neighbor in resampling to WGS84 UTM, as it preserves the original pixel values more rigorously.
5) Edit the parameter file for force-higher-level TSA. This was straight-forward as I could modify my existing parameter file for Sentinel-2, with small modifications.
6) Run force-higher-level to create the spectral-temporal metrics.
The results look very nice visually. The pixels also align nicely with my Sentinel-2 data in the same cube.
So far, I have done this only for my S1B descending data, so I have to still do the same for my S1A data, which are all in ascending orbits. I will have to see if it makes more sense to calculate the spectral-temporal metrics for the whole dataset, or separately for ascending and descending.
As a conclusion, it is very much possible to create a data cube of Sentinel-1 data in Force, and calculate spectral-temporal metrics out of it, but at this point my workflow requires quite many manual steps. I am sure it could also be simplified and improved, for instance the scaling step (3) could be included in the SNAP processing script.