There were three Grenz regiments identified as Wallachian - two from Transylvania (1st and 2nd Wallachian) and one from Banat (Wallachian-Illyrian). could it be one of these? The 1st and 2nd Wallachian were later known as Siebenburgen-Romanische (or something similar).
The only other Wallachian unit I spot for 1792-1814 in a quick flip through Wrede is:
Siebenburgisch-walachisches Frei-corps (April-Aug 1792) - this one was merged with some Serbian frei-corps in August.
Do any of these match? What date does your reference come from?