I accidentally found a unified method of getting and setting sub-object selection for an Editable Poly objects and Edit Poly modifier. The main problem is that the object must be selected.
I found it in PolyTools.ms script(maxRoot\stdplugs\stdscripts\PolyTools.ms), so only 3dsmax 2010 and above can use it.
I am pretty sure that this comes in 3dsmax with PolyBoost integration.
(
-- get vertex selection as bitarray
PolyBGetSel 1
-- get edge/border selection as bitarray
PolyBGetSel 2
-- get face/element selection as bitarray
PolyBGetSel 3
-- set vertex selection
PolyBSetSel 1 #{1..20}
-- set vertex selection
PolyBSetSel 2 #{1..20}
-- set vertex selection
PolyBSetSel 3 #{1..20}
)