In the "pre-1.0-BSPlayer" versions a commandline like e.g.:
bplay "my movie.avi" "my subtitles.txt" -fs
works.
In version 1.0 this feature doesn't seem to work yet.
(I tried: b
splay "my movie.avi" "my subtitles.txt" -fs).
So we'll have to wait till the creator of BSPlayer will have re-enabled this feature.
If its very important for you, you could always use the last "pre-version-1.0" build 501 till then.