Resolution:
One way to achieve this could be:

1. Create a temporary attribute say "TEMPVAR" in Master Catalog.

2. Set this attribute to some value in the SOAP Request e.g.:

&ltEntityData>
&ltAttribute name="PRODUCTID" type="string"&gtP1</Attribute>
&ltAttribute name="PRODUCTIDEXT" type="string"&gtPE1</Attribute>
&ltAttribute name="TEMPVAR" type="string"&gtsomeValue</Attribute>
...
</EntityData>

3. Access it in the rulebase as any other catalog attribute:

&ltconstraint>
&ltname&gtAssignUOMToTEMPVAR</name>
&ltdescription&gtAssign UOM To TEMPVAR</description>
&ltusefor>
&ltvar&gtUOM</var>
</usefor>
&ltaction>
&ltassign>
&ltvar&gtUOM</var>
&ltvar&gtTEMPVAR</var>
</assign>
</action>
</constraint>