I love tiny lenses, but I haven't found anything that is smaller than the Samyang 35mm 2.8 that also covers full frame for Sony's E-Mount. Is there any option I have that could make an a7r2 jacket pocketable? Manual focus is fine, it just needs to cover full frame and be tiny.
The problem is that sensors hate it if light hits them in all too angled a fashion, so short focal length lenses that are simply very close to the sensor and project light in a "wide funnel" fashion can create surprises (eg color vignettes or very bad corner performance).
The most radical option is using an 28mm or 35mm rangefinder or enlarger lens of true prime design (not retrofocal) on a short helicoid adapter (which makes it a partially collapsible lens) - but do expect said surprises, these setups border on the experimental.
The Agfa Ambion 35 f4.0 is interesting here (same that I used for a demonstration in Do lenses affect color fidelity? ... careful, that test shot is not full frame) - but difficult to adapt.