You actually can do this, although the solution is tedious to set up, and can be confusing to devs who are not aware of why it was done. (so if you elect to do it document it thououghly!)...
Create two structs, called say, MyInt, and MyDecimal which act as facades to the CTS Int32, and Decimal core types (They contain an internal field of that respective type.) Each should have a ctor that takes an instance of the Core CTS type as input parameter..
Make each one implement an empty interface called INumeric
Then, in your generic methods, make the constraint based upon this interface.
Downside, everywhere you want to use these methods you have to construct an instance of the appropriate custom type instead of the Core CTS type, and pass the custom type to the method.
NOTE: coding the custom structs to properly emulate all the behavior of the core CTS types is the tedious part... You have to implement several built-in CLR interfaces (IComparable, etc.) and overload all the arithmetic, and boolean operators...
