It would already be enough - at least for the PvE-server - if all nations would start out with own ports and a capital (Even though some might protest due to historical inaccuracy, but it's the PvE-server, so...), so they could use the admirality for rewards. Currently, there's no point in playing as Prussia, Poland (Which I am in) and Russia.
And to claim the rewards, you have to ask players from other nations to get them for you.
If no ports for all nations are an option, then how about making admirality accessible to foreign nations? That would be still incovenient, since you need an Indianaman to ferry the ship notes back, but the easiest solution.