A great mercenary general has to have a great sword, thats all there is to it. 
With a bastard sword being a 2handed martial weapon or a 1 handed exotic, it pretty much just represents the additional training needed to wield it in that style. Commanders, especially the really high up ones often seem to have special weapons that set them apart from the average Grunt or special training and combat prowess, especially during times in warfare where the commanders engaged in dangerous combat similar to their men. I’m reminded of General Patton having those pearl handled pistols he carried with him all the time.
I would immagine though that such wonderfull arms were probably crafted for Daene after he became the Black Baron, as opposed to when he was a mercenary commander. Mainly because they are so fancy, and because they have the symbol of the Black Hart, almost as if they were a gift to commemorate his achievements in forgeing a nation. And I am sure that after he did what he did he probably had some great sword moves like the likes of which the people have never seen.
Another funny thing, In my campaign, one of my players is a Rjurik Druid who carries around a fancy sword that he’s not proficient with just because it is a symbol of the station of his political office. And he blew a decent amount of money having it crafted mainly for the role play value of flaunting himself at court.
Ultimately though, you can make it a longsword and it wouldn’t be out of place . . . If it was a dang fine longsword. I’m just thinking though that since Bastard Swords in 2nd and 3rd edition really aren’t all that different from each other other than the fact that they require a feat in D20 to wield 1 handed, it’s not an issue that I, personally in my own opinion/choice/flavor felt would need any alteration to be converted. However, it could, and that would be fine too.
I like what you are saying about playing off the fact that Roesone, out of all the nations, seems to be the most egalitarian between the classes. As the wording of his arms stands as written, with the gold filigree platemail when common soldiers probably wore padded/leather/chain seems to contradict that.
If you wanted to throw a twist, you could kind of do like Indiana Jones and the Last Crusade, in the scene where they were in the room trying to choose the holy grail. You could have the fancy shiney stuff be the false armor, and maybe anybody who wears it into battle thinks they will get those bonuses but it is really “armor of missle attraction” or some cursed item like that. The “Real” arms could be like common soldier or conscript’s gear, maybe a bit fancier in stitching, but when Donned, it could glamour into really fancy armor, or just have some sort of added charisma bonus to let people know that the person wearing it is the chosen decendant of Roesone. This is by no way canon, just a possible way of handling it that might be fun that popped into my head after reading your post.