This isn't something I get involved in, but your question looked interesting. Two thoughts:

1) Doesn't that ASTM standard (in A3.4) call out 1500 HV10 (Vickers microhardness) - the hardness indenter for that scale is a diamond (inherently harder than the carbide).
2) Could you get around need to self-verify hardness if you purchase an assembled ball/indenter? A quick check online suggests those can be supplied with certification.