There is an active participant who is not a 5% owner, and turned age 70 ½ in March of 1999.
The plan document has always required all active participants to start minimum distributions in accordance with the “old” rules and has not and will not be amended to allow participants to defer.
The plan document allows in-service withdrawals after age 65.
The minimum distribution for 1999, to be paid by April 1, 2000 was calculated to be $2,000. The participant took an in-service withdrawal of $5,000 on March 31, 2000.
The minimum distribution for 2000, to be paid by December 31, 2000 was calculated to be $1,500. The participant took an in-service withdrawal of $2,000 on December 31, 2000.
What amount is eligible for rollover from the 1999 distributions?

From IRS Notice 97-75:
Q-9: If distributions are made under a plan to an employee (other than a 5-percent owner) who did not retire before January 1, 1997 from employment with the employer maintaining the plan, is any portion of a distribution made after attainment of age 70 1/2 a required distribution under section 401(a)(9) for purposes of section 402©(4)(B)?
A-9: (a) General Rule. Section 402©(4)(B) provides that a distribution is not an eligible rollover distribution to the extent that it is required under section 401(a)(9). As noted in Q&A-6, for purposes of determining the amount of minimum distributions that are required after December 31, 1996, the required beginning date for an employee who did not retire before January 1, 1997 from employment with the employer maintaining the plan is redetermined under section 401(a)(9)©, as amended by the SBJPA. Therefore, whether or not a plan allows an employee who attained age 70 1/2 before January 1, 1997, but did not retire from employment with the employer maintaining the plan before that date, to stop receiving distributions in accordance with Q&A-7, a distribution to such an employee prior to the year the employee retires is not a required distribution under section 401(a)(9). Such a distribution is an eligible rollover distribution unless it is excepted for some other reason. An exception is provided under section 402©(4)(A) for a series of substantially equal periodic payments made for the life (or life expectancy) of the employee or the joint lives (or joint life expectancy) of the employee and the employee's designated beneficiary, or for a specified period of 10 years or more. If an employee's benefit is being distributed in a series of annual payments that would equal the required minimum distribution determined in accordance with Q&A F-1 of section 1.401(a)(9)-1 of the proposed Income Tax Regulations, then the series of payments will be considered a series of substantially equal payments over the life (or life expectancy) of the employee or the joint lives (or joint life expectancy) of the employee and the employee's designated beneficiary, or for a specified period of 10 years or more, in accordance with Q&A-5 of section 1.402©-2 of the Income Tax Regulations. Therefore, payments under such a series of payments are not eligible rollover distributions.

I think the excerpt from Notice 97-75 answers your question. The proposed regs at G-1A in (B) of the answer indicates that amounts distributed in a second distribution year are treated first as the current year distribution and then as the remainder of the first year distribution. Thus if the distributions were required, the $5,000 payment would have been comprised of the 2000 distribution ($1,500) and the 1999 distribution ($2,000) and the excess ($1,500) would have been eligible for rollover.

Since the active participant is still working, the distributions aren't required and the entire $5,000 would be eligible for rollover. If that rollover wasn't accomplished within 60 days of the March 31 distribution, then nothing is currently eligible for rollover from that payment.

The December 31, 2000 payment would be eligible for rollover (as I read the proposed regs) even if he were retired because the 2000 required minimum had already been paid.

There are some strange rules associated with minimum distributions and withholding.(and thus eligible for 'rollover'
without additional info, I will 'fudge' some numbers for an example.
minimum distribution based on joint life, age 70 and 70 for spouse, or divisor = 20.6.
Thus original balance = 2000 distribution * 20.6 = 41,200.
But ee could have calculated minimu distribution based on single life = 16, or min distrib of 41,200/16 = 2575.
Thus the amount not eligible for rollover could be treated as 2575 rather than 2000.(Despite the choice used for calculating the minimum distribution1)