To export syslog information using IPFIX you want a Syslog to IPFIX mediator. General information about IPFIX medaitors can be found in draft-ietf-ipfix-mediation-protocol.
There is no restriction on sending strings like syslog messages using IPFIX. There are not currently any standard Information Elements (IEs) for syslog messages. IPFIXify uses proprietary/non-standard IEs for syslog info.
Just to be clear by proprietary I really mean "not necessarily public" or "not standard". A few exporters play their proprietary IEs very close to the vest, but details about most "proprietary" IEs is available if you look. Adding proprietary IEs is a great way to start experimenting with innovative exports. Standard IEs can also be requested by anyone (pending expert review). Every IPFIX exporter I can think of has at least some proprietary IEs.
An other point to make here is that IPFIX allows you to mix and match standard and proprietary IEs (not just send "totally proprietary" IEs). This is important because a collector can provide some reporting using the IEs it knows about and ignoring the others.
I'm not sure what you mean by plain vs. structured IPFIX?
I think that covers all your questions.