Just in case anyone else ever needs a #pseudorandom #Guid generator for #dotnet, here's what I finally did:

private const int guidSize = 16;
private const int guidStrLen = (guidSize * 2) + 4;
private readonly StringBuilder guidBuilder = new(guidStrLen, guidStrLen);

private string CreatePseudoRandomGuidString(Random r)
{
var bytes = new byte[guidSize];
r.NextBytes(bytes);

// Generate a valid UUID-v4 as per RFC 4122, section 4.4
bytes[6] = (byte)((bytes[6] & 0xfU) | (4U << 4)); // version 4
bytes[8] = (byte)((bytes[8] & 0x3fU) | 0x80U); // variant 1

// Format as a string. We don't use the System.Guid class here
// because it uses some mixed endian internal format which might
// not be portable.
guidBuilder.Clear();
for (int i = 0; i < guidSize; ++i)
{
guidBuilder.Append(bytes[i].ToString("x2"));
if (i % 2 != 0 && i > 2 && i < 10) guidBuilder.Append('-');
}
return guidBuilder.ToString();
}
To get a reproducable sequence of pseudo-random Guids, just seed the Random instance with a fixed value before calling this the first time.

Sometimes I think the #10PRINT algorithm can be used as a kind of #UlamSpiral to prove the quality of a #PseudoRandom number generator ...