Can more insulation be blown in between the pine ceiling and the roof?
Blown? No. Sprayed? Yes.
Most likely, your roof has blocking between the eaves with insect screening, and then fiberglass or rock wool insulation laying on the cedar ceiling and an air gap between the insulation and the roof decking. You can shine a flashlight in through the insect screening or remove a piece of blocking to inspect.
If you're lucky, you have R-11 nominal and R-5 effective in your roof. That's way below modern standards for exactly the reason you specify: the ceiling is blazing hot in the summer. It's also where much of your heat is leaking in the winter. When the temps and humidity drop in the winter, the cedar contracts and even more warm air escapes through the ceiling.
If that were my house, I'd be aiming for an unvented cathedral ceiling for two main reasons:
- unvented attics don't suck in embers from nearby fires, lighting your roof on fire
- that previous venting space can be used for insulation
In order to get away with an unvented ceiling, you must have an air-impermeable insulation layer directly beneath your roof decking. If you don't, that warm humid indoor air will contact the decking. When the decking temps are below the dew point the moisture will condense. Nothing that can be blown into that space is air-impermeable.
The only code compliant way I know of to insulate that cavity is to retrofit with a minimum 2" depth (Climate Zone 5) of closed cell spray polyurethane foam (ccSPF) in full contact with the roof decking.
Several spray foam vendors (hint: search on Amazon where you can [mostly] trust the reviews) offer a metered dispensing system that lets you fish the dispenser hose into the cavity, withdraw at the prescribed rate, and completely fill the cavity. ccSPF is fairly pricey but so is tearing off a roof or ceiling. If you do it yourself, ccSPF is likely to be the most cost effective long-term solution, since it packs the maximum R value into the space (R-7 per inch).
If I had a home in Eastern Washington, I'd also be budgeting for that steel roof. If you've only got one layer of shingles, you can lay sheets of polyiso directly on the shingles and install the steel roofing over the polyiso. Having prevented embers from getting into the ceiling, and then preventing ones that land on the roof from contacting a flammable material, your house is far better off than most.