Yes, you can. Stand against a wall, put your arms up above your head, straight over, keeping elbows close to ears at all times. In this elongated position, keeping your butt on the wall, perform what would be a "crunch" maneuver, curling down and over just the upper torso (not "folding" your body at the hips). The length of your arms staying up and stable add "resistance" to this. I do this at work, it actually does place tension on the muscles, especially if you're a beginner.
The ultimate transverse exercise is kegals, which can be performed by men and women alike, are discrete and you can do them anywhere, anytime. You should also always "tighten your pelvic floor" when doing
abdominal exercises as a general rule. Most lifters encorporate this pelvic floor tightening on
heavy squats. The TA is very deep, runs to the "core of your core," you might say.
Start there.