I own two tourers which I normally relubricate their aluminium alloy seatposts every spring with high quality grease. Due to moving to Cambridge and the coronavirus outbreak, both cycles are being kept safe 10 miles away but if I miss this lubrication will their seatposts become seized?
No they won't. It is often enough to lubricate once when the bike was built unless it is used/kept for a very long time. Especially if the bike is kept in a dry place. It is not necessary to lube it once a year. The lube stays there and protects the interface for a long time. If you pull a bike that hasn't been used for ten years from a shed, many things could be seized but even then seizing of the seat tube is far from guaranteed. It is even unlikely if it was well conserved before the storage.
Interface of two different metals could change it due to electrochemical potentials and rusting. I assumed aluminium alloy on both sides or steel on both sides.
Just do it the next year or when you get to your bike.
Once the seat post to seat tube has been lubed, so long as the post hasn't been moved a lot, there truly is no need to lube for a very very long time (many years).
Carbon fiber forms a natural galvanic reaction with aluminum but carbon tubes will have an epoxy layer keeping the carbon from the seatpost.
Titanium has a natural tendency to seize with all types of seat posts so shops typically apply an anti-seize compound which is thicker than a grease, but I have several Ti bikes and have had no issue with using grease on all the various interfaces (bottom bracket, seat post, derailleur hanger, etc etc).
Aluminum & steel will also seize IF the frame corrodes. This is where a thin coat of grease saves the day as it prevents the corrosion for many years. I wrench at a bike co-op where we get a lot of vintage bikes from the 60's through 90's. Many of the bikes have been left outside to rust for decades but if there's grease on the seatpost, bottom bracket, and quill stem all of these easily come apart. So the message here is to grease ALL the connection points and not just the seatpost. Yes this also includes the threads for water bottle mounts, fender/rack mounts, derailleur mounting bolt, and if the bikes have a quill stem the pull it out and grease the threads of the bolt as well as the steerer tube where the quill stem will attach itself.