If you want to Obliterate, as the main thing...
#show Howling Blast
/castsequence Obliterate, Obliterate
/cast Frost Strike

Then have Howling Blast, on another keybind...
Possibly use a video feed, to watch the button for the macro.
And whenever the HB key lights up, because of the Rime Proc, push your alternate keybind for the HB.

Or modify the above macro for mod/no-mod:
#show Howling Blast
/castsequence [nomod] Obliterate, Obliterate
/cast [mod] Howling Blast
/cast Frost Strike



Obliterate = 1x Frost, 1x Unholy runes.
And we have 2x Frost, 2x Unholy, and 2x Death (Whatever works best).

Or if you want the least management...
Just ignore the HB portion, and go with a straight Obliterate.
Possibly two DPS keys, Obliterate = single targets and Howling Blast = AoE/Packs/Swarms...