Adding unique hardware ID to the message part of SLIC table will make no difference, as MS issues certs for the OEMID and RSA Key. If say, MS was to issue certs for each unique hardware id and the OEM would have to create a unique SLIC table for each machine manufactured. It would be a huge problem to both MS and OEMs to maintain all the information, but it would definitely prevent activation exploits.
Certificates would remain like now - one for each vendor, as they verify OEMID and public key only, but SLIC would be specific for each model or model line with almost the same harware, or for all models with the same MB, but i think even now it's this way for Lenovo, for example. "Unique" here i mean not for each machine (like it is with retail activation), but for each series/model. But yes even with that it would be a trouble for OEMs, probably that's why it wasn't implemented.