126.825
At 45 nm and FL70 I might well need descent fairly soon (my ideal from 7000' aal is 50 track miles, if QNH is high and the airfield not high then I might need 35 nm as an absolute minimum to descend safely), and it is perfectly acceptable to leave controlled airspace. At many airports I go to I need to at some stage anyway, because the airport itself is outside controlled airspace. Unpressurised aircraft have a limit to the rate of descent, especially if they have passengers on board. Some light twins can be very difficult to slow down without damaging the engines once a certain descent gradient is needed.
So while the call might be non-standard, the aircraft is also non-standard and the request is perfectly compliant with all relevant rules.
In fact, one of my pet hates is being told I can't descend because it would take me into uncontrolled airspace, I must wait x miles which means I am going to have to request extra track miles (or if I am flying without pax I have to use excessive rate of descent). Some controllers allow me to descend, so it isn't unheard of; that is why I ask.