I think (I hope) upstream will add these options in the future releases, but for now we need to patch networkmanager-openvpn to get openvpn tunnels that need MTU adjustment to work. I found this patch on launchpad https://bugs.launchpad.net/ubuntu/+source/network-manager-openvpn/+bug/112248/comments/26 the original author is Magnus Kulke, I simply strip the strictly debian/ubuntu related part.
Created attachment 234091 [details] networkmanager-openvpn-0.8.ebuild
Created attachment 234093 [details, diff] files/fragmentation_options.patch
Hello Laurento, please try to comment/attach a diff to the current ebuild, this is easier to handle and less data to be reviewed by the maintainer, Tanks for the report! Michael
Created attachment 234383 [details, diff] ebuild diff Sure! here you are
The new version net-misc/networkmanager-openvpn-0.8.1 now in portage supports these features natively.