In fact, you can show out-of-gamut colors even if there is no perceptual intent table in the colorspace. Even more, the out-of-gamut warning will be more accurate when using relative colorimetric.
The way out-of-gamut warning is usually implemented (at least in LCMS and in my own image editor, for RT I’m not 100% sure) is to make a round-trip ICC conversion of the type working_ICC -> output_ICC -> working_ICC, and compare the initial and final image data. If there are differences, it means that some channels have been clipped in the first conversion, i.e. some pixels are outside of the output colorspace gamut.
If you use a perceptual intent in the first conversion, colors near the gamut boundary (but still inside the gamut) will be slightly distorted, and will give differences in the comparison which are somehow uncorrect - think of this as “false positives”.
If RT is providing, as I expect, well-behaved matrix profiles for output, there is nothing one can add to make them “support standard colour management and processing”. They already do, and perceptual intent is simply not foreseen for matrix profiles in the CM. On the other hand, matrix profiles provide the most accurate conversion between one colorspace and another, because the conversion is based on matrix multiplications and not on LUT interpolations (whose precision for example depends on the number of points in the 3D LUT).
This is a valid request, although black point compensation is not as issue and is supported for all profiles, so there is no reason for including it here. Standard matrix profiles usually have a black level at (0,0,0), so no black point compensation is needed when going for example from ProPhoto to sRGB.