Try the following:
1) Extract subtitles to *.bmp files with subrip (ensure that images are full screen and not just the subtitle area: each file will weight about 200KB).
2) Before exiting from subrip, convert the resulting text file into 'Text-spruce subtitle file (*.stl)' -ignore the advice-.
3) When done, edit the text file and you will see something like this:
...
01:00:00:00,01:00:05:00,file0001.bmp
01:00:10:00,01:00:15:00,file0002.bmp
01:00:20:00,01:00:25:00,file0003.bmp
...
4) Insert the following line before subtitle list:
$SetFilePathToken=<<path:>>
5) And finally change filenames in the list like the following (let's suppose all your images are in folder c:\mysubtitles):
$SetFilePathToken = <<path:>>
01:00:00:00,01:00:05:00,<<path:>>c:\mysubtitles\file0001.bmp
01:00:10:00,01:00:15:00,<<path:>>c:\mysubtitles\file0002.bmp
01:00:20:00,01:00:25:00,<<path:>>c:\mysubtitles\file0003.bmp
....
6) The last step is importing this file into DVDMaestro: perhaps when you double-click over a subtitle you won't like the color palette. Just delete the subtitle track in DVDMaestro, reopen the .stl file and change directly the palette (colors and 'contrast', that is, transparency), and import again from DVDMaestro.