Format Preserving Encryption would work fine. The change I would make is not using it as a 'checksum', but instead just taking the ticket number (which may be a value between 0 and, say, 699) and FPE encrypting that as a 12 digit number). That way, you don't have to worry about 'limited entropy', because breaking it would require either guessing the key (which, if it's 128 bits or more, is just too hard) or randomly guessing and hoping you hit on a valid barcode - in that case, if you issue 1,000 valid tickets (and so only values that decrypt to 0 through 999 are accepted), the probability of guessing correctly is $10^{3-12}$, that is, one in a billion - not real good odds.
The only issue I can see with FPE is that there are no common implementations (which is a shame - I believe FPE is a generally useful tool)
However, there is an alternative that is commonly available, and would work just as well - Message Authentication Codes (MAC).
A MAC works like a signature, except that there are not separate signing and verifying keys - instead, one key does both operations. You'd generate a random key, and give that to both the ticket issuer, and the ticket scanner (just like in the FPE idea).
What you'd do is have the first three digits of the ticket be the serial number 000-999 (or 699 if you have 700 guests). The rest of the digits would be the MAC of the first three digits, converted into decimal (and truncated appropriately). To validate the ticket, the scanner would take the first three digits, and compute the MAC (using the key it knows), and checks that to the last 9 digits.
This gives you the same security as the FPE idea, and MAC implementations are readily available. Of course, with an FPE, you don't have to worry about converting things into decimal (FPEs can work directly with decimal values); standard MACs work in binary, and so to make them decimal, some sort of base conversion will be needed; however base conversion code is far more straight-forward than working FPE code.
There are a number of types of MACs available; I'd stay away from MACs with an IV, and stick with either HMAC, KMAC and CMAC.
One final issue for you to consider (which has nothing to do about your question): one obvious attack would be to take a valid ticket, and run it through a copier, and generate a number of tickets which would all authenticate. If you have a single scanner, you can easily stop that (by having the scanner remember all the serial numbers it has seen before). If you have multiple, and they cannot communicate, well, that's a bigger issue.