Thanks for all the reponses so far!
David, the main dilemma for me with a hybrid mast is that building it during the winter would require a workspace which I unfortunately don't have, so it would overlap with the other modification projects I have planned for the boat[1] in the spring, from April forwards, all of which will keep me from sailing next summer. My current long-term plan is to sail locally (around the Baltic) during summers 2014 and 2015 and be ready for a longer voyage from 2016 onwards.
If I had access to a warm large space during the winter where I could build the mast and cure epoxy, an alu/wood hybrid would be a more attractive choice. With a tapered steel pole construction is simple and fast, basically a matter of finishing it and building a masthead fitting which shouldn't take much time. And it's slightly less expensive than a hybrid.
If a 10 meter hybrid mast would be in the 70 kg ballpark and we add 30-40 kg for sail/battens/yard, we're looking at 100-110 kg for the hybrid vs 120-130 kg with steel. Not a huge difference, even if weight aloft (or lack thereof) is important. Or..?
In any case I don't need to make a final decision right now. I will construct the sail during winter and if I can get a hold of a warm space to build a hybrid mast I'll consider it, otherwise I'll just go with steel. I can always upgrade later. In a perfect world I'd prefer an all-aluminum mast but that will have to wait until I'm somewhere in the world where tapered aluminum poles are more readily available.
By the way, I'm curious as to why you think a half-barrel shape would be better than a straight taper?
Arne, the rig has to be offshore-capable.
Bottom line: Yes, a steel or solid wooden mast is on the heavy side and probably suboptimal, but it will most likely have to do for now. The general opinion here seems to be that it will work all right for my boat. And reading the discussion about heavy masts[2] it might even be somewhat of a Good Thing.
[1] Spring 2014 projects: Last spring I built a double berth in the forepeak and now with the junk rig it's going to be ruined with a mast in the middle of it, so I'm planning to build a new one behind the companionway stairs, which requires shrinking the cockpit footwell and raising the bridgedeck. I also did a hasty bottom job to be able to get the boat in the water as soon as possible and it will likely need to be redone. The deck will also need a bunch of work to accomodate the JR mast; reinforcements, partner, move the hatch, etc.
[2] Why try for a light mast (stability, mast weight and rolling):
http://junkrigassociation.org/technical_forum?mode=MessageList&eid=1313268