TeT,
Good start, but you'll be aware that the moon phase alone doesn't determine the actual usable light levels. Cloud, atmospheric pollution, cultural lighting etc all play a part too. Sometimes a flight can be made with no moon but purely on cultural lighting (UK Police have this option written into their "Visual Contact Flight" night rules, it's the pilot's own go/no-go decision).
As a point of interest, in UK, "filing IFR" wouldn't make a difference. Because there is no night VFR. The rules here state in essence that outside of CAS, at 3,000ft or below, IFR are deemed to be met if sufficient visual cues are available to carry on safely. Essentially the same as if the flight was flown under "Special VFR" rules in a control Zone, although the term is avoided in this context.
Obviously, the problem is how to know if sufficient visual cues will be maintained along the whole route.
Having the ability to abort "Night VFR" and pull up to MSA in a properly equipped IFR aircraft is surely safer than not. The worst case is having to "suck it and see" in marginal conditions, in a non-IFR compliant aircraft then fly into reduced visual cues, with no easy get out. If NVGs are in use, and they lose effectiveness due to reduced available light levels or IMC, this is when even highly skilled/experienced pilots can lose it and buy the farm. Night disorientation is very easily found in an unstabilised aircraft. This is why we (RAF SH) fought hard to get the night cloud and vis limits correct. The limits for NVG ops had to also allow "unaided flight".
Thankfully, in UK at least, there is nothing to prevent a properly equipped and trained helicopter pilot alternating between the "Visual Cue by night IFR" (my term) and the "Full IFR" as he sees fit. As long as he can obtain visual cues before he descends below 1,000 feet above the highest obstacle within 5nm of the aircraft, it remains legal and safe. Obviously, provided accurate situational awareness is maintained.
These days, in my small part of aviation, this essentially is how I operate much of the time by night. Sometimes a SVFR departure can be made, followed by an IMC transit, then an ILS to an airfield requiring a bit of a drive for the passengers. Obviously, the icing level may prevent an IMC climb to MSA and that is when finer judgement is needed. Sometimes a "no-go" decision will be made on the ground. Sometimes, that decision will prove in retrospect to have been over-cautious and leave me open to criticism (and it has), but as we all know, it's so much safer than the ultimate penalty paid for being under cautious...
Here it isn't a requirement to file a written flight plan for IFR provided I remain clear of Class A airspace. Special VFR entry to any control zone can be requested on the radio. No guarantees, but procedures are in place to allow it, day or night.