"German main sheet" is a poorly defined term that means different things to different people.
What the OP seems to mean is a coachroof
double mainsheet, one on each side, instead of a traveller. This can achieve the same trim that a single mainsheet with a traveller can, but unlike a traveller you lose the trim, while a traveller retains the sail trim when moved on the track. Found a nice sketch of it here:
http://www.pearsonariel.org/discussion/attachment.php?attachmentid=8705&d=1354456892I've sailed on a 45 Cruiser with such a double mainsheet and found it much more annoying in use, as a traveller keeps the trim when moving and the double sheet system does not, plus you need to fiddle with both sheets when sailing downwind. This arrangement does not act as a boom brake at all. The only thing it does is let you do is center the boom and keep it from jiggling around when moored up.
What "
German main sheet" often means is an arrangement with only one continuous line whose ends are led to a winch on each side of the cockpit. This lets you adjust the mainsheet from either winch and is mostly used on large cockpits with twin wheels so you can use the winch near the wheel. This setup either has an additional traveller, or sometimes no traveller at all, and without one and without the double mainsheets you have limited options for going upwind. This doesn't perform the function of a boom brake either, so I have no idea what you mean by that. Illustration here:
http://s300.photobucket.com/user/hectou/media/mainsheet_9.jpg.htmlIf the mainsheet in the cockpit bothers you (something I can definitely agree with), that is really something to think about at purchase time rather than retrofit later. For the double mainsheet conversion, you not only need two very strong eyes, but also two backing plates to mount them to. On my Bavaria, backing plates are glassed in aluminium plates with threads cut in and if those aren't there, you have some big modifications to do, and your coachroof may not be designed to carry these high loads at all. Your boom, as you already described, most certainly is not - mid-boom sheet loads are much higher than end boom and you would also need suitable fittings on the boom. One thing you can do is peek into your ceiling and look for the backing plates (you can see them through the GRP on a sunny day). If they are there, your coachroof would probably be up for it, but you may need a stronger boom (Selden will likely be more useful with advice than Bavaria regarding boom loads).
Personally I like my mainsheet out of the cockpit
and at the boom end and that's why I bought a rare center cockpit Bavaria