In-service withdrawal for short-term disability

Our plan is looking into the possibility of adding an in-service withdrawal feature tied to the occurrence of a short-term/temporary disability.  We have not been able to find much guidance in our research to confirm that (1) this is permissible and (2) what parameters can we place on it.  I know the Treas. Reg. 1.401(b)(1)(ii) allows for in-service withdrawals of funds after a "fixed number of years, the attainment of a stated age, or upon the prior occurrence of some event such as layoff, illness, disability, retirement, death or severance of employment."  Would a short-term or temporary disability (that would not qualify for the exception to the 10% early withdrawal penalty if the worker is under 59 1/2) be an event that would allow for an in-service withdrawal?  (We would only allow withdrawal of employer contributions that are vested and put a cap on the amount that could be withdrawn, plus require proof/documentation for the short-term disability).  If anyone has added this type of feature or can provide references to any IRS or DOL guidance (whether articles, PLRs, Rev, Ruls., etc) that would be great. My research hasn't been very fruitful (although I did come across at least one plan that incorporates this feature) but not much more. 

Why don't you provide disability insurance instead of allowing people to use their retirement money?

This is a union plan and we do have A&S benefits under the Welfare Plan, but the trustees were interested in ways to allow access to funds to supplement since this would not increase costs to the plan and the employees wouldn't need to pay premiums for another plan. 

You can provide in-service w/d for non-restricted funds (i.e., those not restricted prior to 59 1/2, hardship or separation, such as 401k) on whatever uniform and nondiscriminatory basis. However, it gets administratively messy if you allow from partially vested accounts.