I don't have a Lensbaby but I have a Nikon D90 and I run into the same problem when attaching my camera to my telescope, or simply putting duck tape over the lens mount and pricking it to make a pinhole camera. The consumer Nikon's (before the D7000) simply won't meter.
You have three choices. You can get a handheld meter. That will be fairly accurate and may turn out to be the fastest. Or you can "chimp" the histogram. You keep checking the histogram across multiple tries, adjusting shutter as you go. When the histogram looks "right" (whatever that means) you're done.
The third way is to work from an Exposure Value table. This requires carrying two charts (which you can just print double-sided.) The first is an EV table that gives you the F-number and shutter combination for a particular EV. The second is a list of common EV values for various lighting conditions (a list that you'll continue to add to with experience.)
Here's an example of an EV table...
http://en.wikipedia.org/wiki/Exposur...amera_settings
Here's an example of the EV values...
http://en.wikipedia.org/wiki/Exposur...xposure_values
This method starts by you evaluating the light on your subject. For example...from the EV list above, if you're in an area of open shade and conditions are clear sunlight, then your EV is 12. Next, you check which aperture you have in your lens, and check the EV table for that F-number and EV. So if you think the lighting is 12 EV, and you have an f/2.8 aperture in your Lensbaby, then the shutter is 1/500s. (If you're not shooting at ISO 100, then simply move down a row for every doubling of ISO...so ISO 200 is the next row, ISO 400 is two rows down, etc.)
This method is fairly straight-forward and it teaches exposure well, because you see exposure as more than just a combination of aperture and shutter. You'll eventually memorize part of the table and learn to deduce the parts you didn't remember, and you won't need to carry the charts anymore. You'll be able to set exposure fairly well just by looking at the light...which would be quite an impressive skill to have in an age of intelligent metering modes that do everything but press the shutter for you (oh wait...they now have "smile detection" mode...that presses the shutter for you.)