There is an extended SRT format specification. The link to visualsubsync merely confirms that this very program only supports the standard spec. It also implies that there indeed is support for coordinations and there is:
You need to place coordinates for the pixels behind the time code like that:
00:10:30,796 --> 00:10:33,799 X1:117 X2:619 Y1:042 Y2:428
<font color="#ffffff">Any kind of text</font>
The example was taken from the following link:
Your player, encoder, filter etc. might not support the extended SRT specs, though.