writes:

So you just fly along in Class G and unilaterally declare yourself to be
flying IFR?

If both you and the aircraft are certified for instrument flight, yes.

See FAR 91.173.

Class E has the exact same provision by definition since by definition it
starts at 1,200 AGL.

No. There is no guarantee that you will not encounter obstacles or terrain
simply because you are in Class E airspace. On an airway, however, as long as
you remain within the volume of the airway, at or above the minimum altitudes
prescribed for that segment of the airway, you won't hit anything.

By definition, Class E starts at 14,500 feet, although in practice the floor
is usually lowered to 1200 feet AGL. However, it can also start at the
surface, which means that you can hit obstacles extending into Class E
airspace.

The point is absent a radar altimeter, you can not tell from navaids
whether or not you are above the lower bounds of either Class E or a
Victor airway.

Victor airways have published minimum altitudes, which you can heed with an
ordinary barometric altimeter. Above these altitudes, you're safe.
Ground-based navaids do not provide altitude information. GPS altitude is
generally unreliable.

What provides obstacle clearance during the day is looking out the window
and at night paying close attention to the Sectional.

I hope you leave generous margins in the latter case.

Or you can fly IFR and use the published minimum altitudes, which are very
generous (so much so that they can be problematic for tiny airplanes in some
areas of the country).