Can you force SQL Server to use an index on computed column for a commutative operation?
As far as I know, your only options are to create the second computed column, contact the vendor of the software to ask for a fix, or to submit an enhancement request to Microsoft to get better support for your scenario. On the surface, the functionality that you're asking for could be viewed as simple: why can't SQL Server figure out that the expressions are equivalent when they obviously are to a programmer? However, it requires at least all of the following:


SQL Server needs to know that DATEADD(minute, ValMin, DATEADD(hour, ValHour, ValDate)) is equal to DATEADD(hour, ValHour, DATEADD(minute, ValMin, ValDate)). There are a lot of date equivalences that SQL Server isn't yet aware of, and this is one of them.
The query optimizer would need to look for commutative matches during computed column matching.
The query optimizer would need to be able to find your match during the query optimization process, which is designed to very quickly give you a "good enough" plan.


I am sympathetic to your problem, but my guess is that the scenario that you're describing just isn't common enough for Microsoft to make improvements in this area, especially when there is a straightforward workaround that could be accomplished through changing code or by adding another computed column to the table.