For your specific result, I have no suggestions or references.
In general, I suggest including a Proof Hint: or Proof Sketch:, along with a note to the editor that you would like assistance in finding a proper reference. The hint should occupy little more space than a full bibliographic citation, and you can prepare a (possibly never to be published) appendix which contains sufficient details of the proof and a summary of your efforts in finding a reference, should someone call you on the (truth of the) statement. This should be doable in a short time and allow you to delay/defer/circumvent this issue. If this is not doable in a short time, then you might rethink its use as a motivating statement, as it may be more of a distraction than motivation. (You can also solicit the editor's opinion on how to handle this issue.)
Gerhard "Ask Me About System Design" Paseman, 2011.04.11