How to select a diamond lap/sharpening stone?

I have some cemented carbide inserts I'd like to touch-up, but I don't have a diamond grinder. Despite it taking a long time, is there any reason I couldn't use a diamond lap for this? If it's possible, where should I start?

Let it suffice to say, that sometimes I have a LOT of idle time to kill.

We have discussing sharpening Carbide inserts we use for scraping. Many members have designed and shown home made carbide laps. Made from used motors used in sewing machines, exercise machines that can be bought for pennies at Good Will or I suspect on CrigsList or Ebay. You can but diamond wheel disks on Ebay for something like $15.00 each, but many buy 3 or for, 260, 600, 1200 and 2000. Probably make one for under $100.00.
Goodgle Glendo replacement practical machinist and you can see several posts on this.
http://www.practicalmachinist.com/vb/archive/index.php/t-321439.html
As far as the diamond handlap I have never used one to touch up turning inserts. I have tried on scraper blades and not being able to keep the angle the same. I like them for a file and he flat box to clean and flatten sharpening stones.

I do that all the time. A woodworking catalog has those flat plates (2" x 6") in fine, medium, and coarse. The first ones I purchased are still able to cut and I use them on brazed carbide bits. Since there are only three flat surfaces on those bits it is relatively easy and it really extends their life. On inserts I have done touch up sharpening. Good old spit is my usual method for wetting the plate.

The diamond stones of today do not appear to be of the same quality compared to 25-30 years ago.

Diamond cuts carbide pretty fast and easy, you might be surprised, its gotta be on a par if not faster than sharpening wood tools on a oil stone. DMT if there still around make nice laps, i find WD40 a nice lubricant - cutting fluid.

Your most difficult task will be holding the insert. With a brazed carbide bit it is no sweat because the shank is a couple inches long. A small insert will probably require a home brew method for holding. But it can be done. Almost every time I do sharpening this way I think of the Japanese woodworker squatting on the floor with a set of water stones.

I use a magnifying eye piece to inspect the work but I don't wear a bandana around my head.