Possible to Re-tighten Muscles Without Entire New Tummy Tuck?

Technically, as Dr Placik points out, the re-tightening of the fascia is not difficult and can be accomplished with or without an endoscope.
However, the results can be disappointing.
First, the fascia may actually be tight, so additional sutures literally rip through, not achieving the effect the patient had been hoping for.
The usual reason patients develop a bulge after having originally undergone a tummy tuck is weight gain. That means there is more fat inside the abdomen. A tummy tuck retightening does not remove any of that fat, so tightening of the fascia can be a very disappointing experience for the patient expecting the results of their first tummy tuck.
The best results of the "re-tightening" procedure are with patients who have lost weight (including those who have had a pregnancy after a tummy tuck), where the fascia is legitimately loose, not just bulging because of fat inside the abdomen.
For cases where the bulge or "beer belly" appearance is unsightly, weight loss is by far the best (and healthiest) option for the patient.

If your abdominal muscles are loose because of a diastasis (split of the rectus abdominus muscles), the split and laxity extends all of the way up to your lower chest.
If your doctor attempts to tighten only the lower abdominal muscles through a mini tummy tuck approach the lax upper abdominal muscles will create an unsightly visible bulge above your umbilicus (belly button.)
If this occurs you will need a revsion.
Consult an experienced board certified plastic surgeon for an exam. If the upper abdominal muscles are lax, they need to be repoaired as well as the lower abdominal muscles

As the other doctors have indicated, this can be done with a C-section type of scar.
My question, given your history, is whether or not muscle tightening is what would correct your issues. You could have excess skin, excess fat, or some combination of the two...which will obviously change the approach to get you the desired results.