Writing matroska to an append only stream

Question

I need to write a matroška video file to a stream that only supports an append operation (not network streaming, output is a single MKV file for offline playback). Right now I'm using ffmpeg's libavformat to do the muxing, but the resulting video file is not seekable (in a player) at all.

Going through the matroška specs, I figured out a way to create a seekable (in a player) file work with only one (file) seek operation:

SeekHead 1 (without clusters)
...
Clusters
Cues
SeekHead 2 (only clusters)

After the file is written I need to go back to SeekHead 1 and update it with positions of SeekHead 2 and Cues.

My output files can easily get to tens of gigabytes, so buffering the whole thing in memory is not an option.

Is there really no way to create the MKV without seeking in the output file?