I have not come across such an API. The following function will return the lists of 2 winding transformers regulating the bus you specify by the icont argument.
def TwoWTfmr_RegIcont(sid=-1, owner=1, ties=1, flag=1, entry=1, icont=0):
ierr, [bus1, bus2, icontnumber] = psspy.atrnint(sid=sid, owner=owner, ties=ties,
flag=flag, entry=entry,
string=['FROMNUMBER', 'TONUMBER',
'ICONTNUMBER'])
ierr, [tfmr_id] = psspy.atrnchar(sid=sid, owner=owner, ties=ties, flag=flag,
entry=entry, string='ID')
tfmrs = [(b1, b2, tid) for b1, b2, tid, icnt in zip(bus1, bus2, tfmr_id, icontnumber)
if icnt == icont]
return zip(*tfmrs)
# Sample usage
psspy.case(r"C:\Program Files (x86)\PTI\PSSE33\EXAMPLE\bench2.sav")
[b1,b2,ckt] = TwoWTfmr_RegIcont(flag=2, icont=6011)
print b1, b2, ckt