Hi guys, what you're trying to do here is non easy at 10.0. We're working on some nice functions for the 10.1 release to do exactly what you're asking for.This is still under development and subject to change... but here's what it should look like at 10.1. Then you could put the tlist (below) into your script tool's params.filter.list = tlist>>> from_sr = arcpy.SpatialReference('NAD 1983 HARN UTM Zone 11N')
>>> to_sr = arcpy.SpatialReference('NAD 1927 StatePlane California VI FIPS 0406')
>>>
>>> # extent of San Diego county in from_sr coords
>>> ex = arcpy.Extent(444450.2212, 3599832.1877, 585727.9387, 3707930.3429)
>>>
>>> tlist = arcpy.ListTransformations(from_sr, to_sr,ex)
>>> pprint.pprint(sorted(outlist))
[u'NAD_1983_HARN_PACP00_MARP00_To_WGS_1984 + NAD_1927_To_WGS_1984_18',
u'NAD_1983_HARN_PACP00_MARP00_To_WGS_1984 + NAD_1927_To_WGS_1984_4',
u'NAD_1983_HARN_PACP00_MARP00_To_WGS_1984 + NAD_1927_To_WGS_1984_6',
u'NAD_1983_HARN_PACP00_MARP00_To_WGS_1984 + NAD_1927_To_WGS_1984_79_CONUS',
u'NAD_1983_HARN_To_WGS_1984 + NAD_1927_To_WGS_1984_18',
u'NAD_1983_HARN_To_WGS_1984 + NAD_1927_To_WGS_1984_4',
u'NAD_1983_HARN_To_WGS_1984 + NAD_1927_To_WGS_1984_6',
u'NAD_1983_HARN_To_WGS_1984 + NAD_1927_To_WGS_1984_79_CONUS',
u'NAD_1983_HARN_To_WGS_1984_2 + NAD_1927_To_WGS_1984_18',
u'NAD_1983_HARN_To_WGS_1984_2 + NAD_1927_To_WGS_1984_4',
u'NAD_1983_HARN_To_WGS_1984_2 + NAD_1927_To_WGS_1984_6',
u'NAD_1983_HARN_To_WGS_1984_2 + NAD_1927_To_WGS_1984_79_CONUS',
u'NAD_1983_HARN_To_WGS_1984_3 + NAD_1927_To_WGS_1984_18',
u'NAD_1983_HARN_To_WGS_1984_3 + NAD_1927_To_WGS_1984_4',
u'NAD_1983_HARN_To_WGS_1984_3 + NAD_1927_To_WGS_1984_6',
u'NAD_1983_HARN_To_WGS_1984_3 + NAD_1927_To_WGS_1984_79_CONUS',
u'NAD_1983_To_HARN_CA_S + NAD_1927_To_NAD_1983_NADCON',
u'WGS_1984_(ITRF00)_To_NAD_1983_HARN + NAD_1927_To_WGS_1984_18',
u'WGS_1984_(ITRF00)_To_NAD_1983_HARN + NAD_1927_To_WGS_1984_4',
u'WGS_1984_(ITRF00)_To_NAD_1983_HARN + NAD_1927_To_WGS_1984_6',
u'WGS_1984_(ITRF00)_To_NAD_1983_HARN + NAD_1927_To_WGS_1984_79_CONUS']
>>>