What does it say in the technical rules about maximum plan area/side pod width?
I've seen it suggested in the past that the Lotus 87 and 88 were designed to comply with the proposed WFMS regulations which is why they had narrower side pods than other cars in 1981.
I don't know if that is true but I recall something else about Chapman.
In his book "Grand Prix Story 1980" Austrian writer Heinz Pruller refers to an ultra secred document he had read. It was written by by FOCA on request of the FISA to stipulate how they wanted to reduce corner speeds. the document was dated 18 september 1980. From what I can make up of what Pruller mentions, this might have been this particular document as mentioned in the OP.
FOCA team members had participated in discussions. But one rule that Chapman was fighting for to achieve was to have the "Car volume" to be reduced from 5.5 to 4 Cubic meter. This triggered a number of other constructors who feared that Chapman had made another discovery since ground effects and skirts and were worried what Chapman would have hidden within his sleeves this time.
Such a reduced `car volume` would indeed have been an advantage for a car like the Lotus '86/88 and the later 81.
To tease a bit more. (For Dutch readers who have the book, look at the pages 202 and 203 to verify where the listed below is coming from, for they who own the original German version: this can be found within the last 4 pages of the last chapter.
Pruller mentioned the following points being discussed:
Wheels of 13 Inch: would not be approved by Renault who needs larger wheels due to their larger brakes.
Restricting fuel flow: Would needs Renault's approval but couldn't be too hig because more fuel would mean more power for the turbocharged engines. The eventual suggestion was 50 cc/sec which would allow up to 600 (turbo)hp. Cosworths were on 41.5 cc/sec.
The total air volume within the tires were reduced from 476 to 400 liter. Because the smaller tires would account for some 20 kg weight reduction the minimum weight was to remain at 575 kg.
Mind you, all of this was publised by Pruller already in early 1981!