DONE. That took me hours!!

Solution:
decode64 the attachment, so you get the winmail.dat.
Search for "MtgResp" in the binary. The next byte is the status:

MtgRespP = Accepted
MtgRespA = Tentative
MtgRespN = Declined

To identify the appointment ID you should use a prefix for your
appointment. My script uses:

UID:epluscrm103

So I look for "epluscrm" in the unreadable binary trash and take all
bytes till the next 00-byte.