Hi Mr RB,
Found this thread this AM, and have been intrigued by the responses so far. So between shopping with my wife and sharing our laptop (on the road in Montana), I have been working on your question. I know you said you wanted a passive filter for your design, but the cost and real estate required might be a burden. So I'll present an alternative for your consideration.
As Eric recommended, FilterPro is a very good resource. Also, as Mr Al recommended, a bandpass filter is a very good solution for resolving THD to the minimum level with a squarewave input as I have also found.
So I ran a sims in LTSpice for both a 1kHz square wave and the PIC output you described using a 6th order BPF with a Butterworth response, a Q of 20, a gain of 1 and a bandpass of 100Hz. The filter with a 1kHz squarewave had a THD of 0.237%. The second harmonic at 3kHz was dominate at ~60db below the fundamental, which was to be expected with the squarewave input. However, with a simulated PIC input to the BPF, as I believe you indicated, the THD dropped to 0.0086% with the 1st harmonic at ~84db below the fundamental and the 2nd harmonic ~91db below.
This option would seem to meet your criteria, but is not a passive solution. Hope this is some help.
P.S. I'll attach the sim of the PIC input. It is not the exact 2V P-P output you cited, but with a spare amp you can set the gain at output for the output level desired.
EDIT: Just thought to sub a LM6144B quad for the AD822 dual (cheaper unit cost), both being rail-rail amps and it came up with a THD of 0.0069%. However, the second harmonic did come up to ~88db below the fundamental vice ~91db with the AD822.