It's not uncommon to carefully position several strain gauges so that the total result can be adjusted to give a continuously correct indication. However, I think the designer would probably rather have legs with integrity, rather than be asked to strangely shape them to suit strain gauge placement (look up a "Sting" as used in wind tunnels - works well but you get some rather funny internal shapes).
I'm with the oleo pressure method - and I can't see why a temperature transducer can't be sat next to the pressure transducer. Thereafter the maths should be pretty straightforward.
There's no strict rule on weighing intervals, but I'd be surprised if many aircraft get away with less than a 5-yearly re-weigh. Balance sheets are constantly adjusted the rest of the time as equipment is added / removed and then specific loading sheets are prepared for each trip.
G