I
have seen a lot of creative solutions for this problem.
Some are pictured in the custom gallery. One fellow simply
attached screw eye bolts to the handlebars and used a padlock:

Another
and more elegant solution was presented on this bike I saw
at Cal BMW at the recent spring fest. A ball was mounted
on a post on the handlebar - large enough to accommodate
the helmet d-ring, but not the lock:
.
There
are also some locks made to bolt to the license plate frame
that are available online from <http://www.sporttouringusa.com>.
Another option is a bar mounted lock - it will mount on
the sub-frame bar below the saddle - and you can access
it via the opening in the plastic cover. I purchased one
for my RT at a local cycle shop, but the loop is quite small
and I need to use a steel T-extension to attach my helmet.
Here is a picture of the setup without the plastic cover
installed:

However,
Tom Stuart <cstuart@mba1982.hbs.edu>
of San Mateo California has come up with the best solution
of all. Purchase a BMW R1100GS frame mounted helmet lock
from your dealer and mount it to your RT. It can be keyed
to operate with your ignition key - so you have only one
key for the entire bike. His looks great:
and the installation is pretty straightforward. Tom says
"There is a piece of steel body tubing at one side
of the opening where the lock is shown. To attach
the GS lock to this, I used a conduit clamp and a small
(1"x1"x1/8") rubber sheet. The conduit
clamp goes around the tubing. The rubber piece fits
between the conduit clamp and the tubing to help make it
a tight fit. When the conduit clamp is situated properly,
the hole in it that accepts the tightening bolt is
lined up perfectly for the hole in the GS lock. Find a bolt
that is long enough to go through the hole in the GS lock
and appropriate sized to fasten to the tightening
hole in the conduit clamp. Tighten it down and you're all
set. Remember to get the lock keyed so that it can use the
same key as your other RT locks. Mine fits so that one helmet
doesn't need a t-extension. Two helmets can fit with t-extensions.
Good luck. I really like mine."
Dirk
Jesinghaus <Jesinghaus@t-online.de
> of Germany offers another solution. There is
a BMW cable lock that is available for the LT: "Personally
I prefer a very simple solution: I use these hooks under
the passenger seat. This makes sense once you buy the special
steel wire that comes with the K 1200 LT (BMW part No 46
63 2 347 387, about 8$). It is very comfortable - and you
do not need a second key." It can be keyed to your
BMW ignition key.