Hullaballoo,
'Equivalent Cat A' is not a definition per se but a language construct; it is provided because there are still, in operation, a number of Part 27 twins that were built before the introduction of Appendix C to Part 27 (AS355, Bo105, A109). JAR-OPS 3 acknowledges this fact and permits a route for acceptance of such twins if they meet the set of conditions that are provided in ACJ OPS 3.480(a)(1) and (2).
If you read and understand the text, you will see that it is an attempt at setting the minimum standard necessary to permit these aircraft to operate over a hostile environment. It is in fact the build standard that is provided in Category A certification (and is a minimum subset of the requirements of Part 27 Appendix C - which itself contains pointers to Part 29 rules).
Reading Paragraph 3 of the ACJ will also provide insight into the deficiencies in the provision of performance data in Part 29 - before revision 39. It indicates that (additional) scheduled performance data might be necessary to meet the requirements of Performance Classes 1 and 2 (Subparts G and H).
You will not find 'equivalent Cat A' in Parts 91 or 135 because those codes do not have performance standards for helicopters. Part 27 and 29 have no need for the description as they contain the certification codes for Category A.
As biz-buz has pointed out, the flexible part of the ACJ is in the assessment of engine isolation; this is written in those terms because MG3 of Part 27 contains guidance material for such an assessment.
As has been discovered by biz-buz, it is quite difficult to provide a definitive compliance document for each aircraft - hence the assessment sits in the lap of the Gods. The transition from JAR-OPS to EASA Ops has not made this any easier.
Jim