this is not about support for compression itself. That (and support for UDP) would still be nice to have and is promised for RouterOS v7. My request is on a much lower level and I think it should be pretty easy to implement, even in RouterOS v6.x.
Let's have a look at the openvpn man page first. The interesting information is within the paragraph on --compress option:
--compress [algorithm]
Enable a compression algorithm. [...] If the algorithm parameter is empty, compression will be turned off, but the packet framing for compression will still be enabled, allowing a different setting to be pushed later.
That is where the openvpn client in RouterOS fails already: Even if the server has compression turned off but sends the packet framing for compression the connection fails. In this case adding (or interpreting) a single byte in the frame should be enough to fix the connection. This change should allow to use every TCP server that does not enable compression explicitly.
(Within the openvpn source code look for COMP_ALG_STUB in src/openvpn/comp.h line 40 and src/openvpn/comp.c line 46.)
Any chance to see support for packet framing for compression any time soon?