Question: Can I "add on" gliders to a Sport Pilot ticket in the
same way I would to a ASEL certificate?

The short answer is yes.

In order to obtain the privileges to operate an additional category or
class of light-sport aircraft (and quite a few glider models do meet
the definition of "light-sport aircraft," as defined in FAR 1.1), a
sport pilot must receive appropriate training, a proficiency check,
and related logbook endorsements from authorized instructors, as well
as complete some paperwork. The FAR to reference is 61.321.
Privileges to operate each additional make and model within the glider
category are obtained with training and endorsements specific to that
make and model, per FAR 61.323.

The primary difference between the sport pilot "transition" from the
airplane to glider category and transitioning from PPL-ASEL to PPL-G,
for example, is that with the PPL a check ride with a FAA examiner is
required whereas the sport-pilot requires a "proficiency check" (and
endorsement) from an instructor other than the one who provides the
training endorsement. (Of course training requirements, experience
minimums, and operating restrictions differ as well.)

Anecdotally, I have heard of a holder of a sport pilot certificate
(light-sport airplane) who went through exactly this process to "add
on" a glider endorsement. The endorsements were received, paperwork
prepared, and everything was in order, but in the end the pilot
decided to just go ahead and complete the few remaining additional
requirements for a PPL-G rating.

The page at the following URL contains a good synopsis of the Light
Sport Aircraft / Sport Pilot Rule. http://www.sportpilot.org/learn/fina..._synopsis.html
See the condensed rule: http://www.sportpilot.org/learn/sp_rule.pdf
See complete rule: http://www.sportpilot.org/learn/final_rule.pdf

Also note that part of what BT mentioned is certainly true. The sport
pilot with an endorsement for the glider category will have more
operating restrictions than a pilot with a PPL-G rating.

I've heard rumors about a "light-sport" category of glider, but I'm
interested in the full monty.

It seems that as long as a glider meets the definition of light-sport
aircraft (see FAR 1.1), then it may be flown by a sport pilot who has
received training and a logbook endorsement from an authorized
instructor for the category, class, and make and model of aircraft.
Note that for the sport pilot, training and an endorsement for each
specific make and model of aircraft may be required, as summarized in
FAR 61.323.

Many "older" gliders meet the definition of "light-sport aircraft,"
but it appears that most of the more modern (glass) ships do not,
primarily due to the Vne restriction.

I'm currently working on my Sport Pilot certificate and when
that's done I want to add glider "approval".

Fly safe, have fun, and finally, IANAL.