Hm, in the mean time I figured out some things myself.
The 4 words before the image data are the maximum width and height of the image data and (probably) the width and height of the whole image to be produced, including transparent padding pixels. Probably the two words in front of that are the x and y offset of the stored image inside the transparent target image.
The image data is stored line by line and has the following format:
[x offset] (0x80+len) ... len bytes ... (0x80)
The x offset can be omitted - then it's most probably 0. Obviously it can't be larger than 0x7f, since then it could not be distinguished from the start of line character, which includes the length of the line (0x80 + len).
This also means that a line can't be larger than 0x7f since else it wouldn't fit into a byte.
E.g. the line
0x84 0xAA 0xBB 0xCC 0xDD 0x80
represents four bytes image data (AA,BB,CC,DD) starting at offset 0.
0x03 0x85 0xAA 0xBB 0xCC 0xDD 0xEE 0x80
represents 5 bytes image data (AA,BB,CC,DD,EE) starting at offset 3.
So each line can have a different length, but (excluding start and stop byte) must not exceed the maximum width given in the header (probably for memory allocation issues). The height given in the header equals the number of lines.
If there is more than one animation frame, there is a frame header of 6 words after the image data of the last animation and the image data of the next one. The 6 words are probably x and y offset, maximum width and height of image data stored and width and height of target image.
Between the file header ("SRLE", numberOfFrames) and the first frame header a number of bytes is stored - maybe information for frame grouping or something.
However, I noticed that the 2nd word after "SRLE" is the offset to the first frame's header (relative to the start of the file).
So the file header is:
"SRLE" (numberOfFrames) (firstFrameOffset) ....
Where frames and offset are stored as 16 bit words in little endian format.
And yes, I figured it out by just staring at the bytes
Guess I will write a little tool to show the animation frames and store them into a GIF or something.