I tried your example and got an error:

  error: type capture contains reference to comptime var

I'm not sure how you were suppossed to use it but here's my attempt:

  fn capitalize(tag: []u8) []u8 {
      tag[0] &= 95;
      return tag;
  }

  fn MyType(T: type, comptime tag: []u8, comptime count: usize) type {
      const capitalized = capitalize(tag);
      return struct {
          fn name() []const u8 {
              return capitalized;
          }
          array: [count]T,
      };
  }

  pub fn main() void {
      comptime var hello: [5]u8 = undefined;
      @memcpy(&hello, "hello");

      var v: MyType(i32, &hello, 10) = undefined;
      v.name();
  }

If you allow `capitalized` to be it's own instance then there's no reason to mutate the comptime parameter in the first place, and it can be replicated in C++17.