A non-instrument rated private pilot is flying alone, VFR in VMC. Without violating any FAR's, can he log actual instrument time?
Without question, yes. M. Burritos cited a few examples of situations requiring flight by reference to instruments. A pilot who is not instrument rated, who logs instrument time under such circumstances,should note the circumstances and the reason for logging the time as such, in the remarks section of that particular logbook entry.
Before the airline interview crowd gets to whooping and hollering, can it.

There is no issue here. Further, the FAA has released a legal interpretation specifically addressing the matter. Yes, it's perfectly legal, and acceptable, and no interviewer worth his salt is going to bat an eyelid.
The following excerpt from a legal interpetation by the FAA Chief Legal Counsel addresses the issue:
November 7, 1984
Mr. Joseph P. Carr
Dear Mr. Carr:
This is in response to your letter asking questions about instrument flight time.
First, you ask for an interpretation of Section 61.51(c)(4) of the Federal Aviation Regulations (FAR) regarding the logging of instrument flight time. You ask whether, for instance, a flight over the ocean on a moonless night without a discernible horizon could be logged as actual instrument flight time.
As you know, Section 61.51(c)(4) provides rules for the logging of instrument flight time which may be used to meet the requirements of a certificate or rating, or to meet the recent flight experience requirements of Part 61. That section provides in part, that a pilot may log as instrument flight time only that time during which he or she operates the aircraft solely by reference to instruments, under actual (instrument meteorological conditions (imc)) or simulated instrument flight conditions. "Simulated" instrument conditions occur when the pilot's vision outside of the aircraft is intentionally restricted, such as by a hood or goggles. "Actual" instrument flight conditions occur when some outside conditions make it necessary for the pilot to use the aircraft instruments in order to maintain adequate control over the aircraft. Typically, these conditions involve adverse weather conditions.
To answer your first question, actual instrument conditions may occur in the case you described a moonless night over the ocean with no discernible horizon, if use of the instruments is necessary to maintain adequate control over the aircraft. The determination as to whether flight by reference to instruments is necessary is somewhat subjective and based in part on the sound judgment of the pilot. Note that, under Section 61.51(b)(3), the pilot must log the conditions of the flight. The log should include the reasons for determining that the flight was under actual instrument conditions in case the pilot later would be called on to prove that the actual instrument flight time logged was legitimate.
Sincerely,
/s/
John H. Cassady
Assistant Chief counsel
Regulations and Enforcement Division
As for logging instruction received and PIC, there is no issue at all. No airline is going to balk at logging PIC when you're legally justified in doing so...and you're perfectly fine showing that you were receving instruction while acting as PIC. HOWEVER, many airlines and firms will want to see PIC, SIC, and instruction received add up to equal total time. If I am PIC on a flight in which I am receiving instruction, I log both PIC and instruction received on the particular line entry in the logbook. However, the instruction received doesn't show up in the totals at the bottom of the page. In my logbook, only instruction received which cannot be classified as PIC or SIC shows up in the instruction received totals. Therefore, PIC, SIC, and instruction received add up to total time.
are you saying that technically you have to add up all of the flight time that your instructor spends demonstrating a maneuver (i.e. a hold or approach) during instrument training, in a plane that you are rated in? Then you have to subtract that time from what you log as PIC.
Correct. You may only log as PIC that time which meets the requirements of 14 CFR 61.51(e). If your instructor takes the controls to demonstrate something, or flies for a portion of the time you are in the airplane, then you may not log that time, except as instruction received.