They could add a bypass circuit around their boost converter, which would give virtually 100%[1] efficiency until the booster actually fires up.

Something like the TI TPS61291[1] draws only 15nA in bypass operation, and maybe 85% efficient in boost.

[1] The bypass switch itself will have some resistance, the part below is ~1.2ohm, which might be significant depending on your load.

[2] http://www.ti.com/lit/ds/slvsbx9a/slvsbx9a.pdf