In addition, it's very likely that the Sapling circuit upgrade will substantially reduce memory usage. There are some very rough and preliminary estimates of how much Sapling could reduce the circuit size here. The reduction is given in terms of its effect on proving time, but the memory also scales roughly proportionally to the circuit size. So, a switch from SHA256Compress to Pedersen commitments for all of the hashes, combined with the as-needed key loading @str4d mentioned, might reduce the memory usage to roughly 1.5 * 4/42 ~= 0.14 GB, for example. This is before we've added any circuit support for new Sapling features, though. In any case I estimate the memory usage for creating a Sapling shielded transaction, spending from new addresses, is likely to be less than 0.3 GB (perhaps substantially less; it depends how much time we have to spend on memory optimization). This is not any kind of guarantee or commitment, just my best estimate at the moment.
Edit: improve estimate based on a recent measurement.