DFARS 215.470 requires a CDRL when the Government requests data. "Data" is not defined in the DFARS as far as I'm aware. However, I think you're correct that this requirement applies to FAR Part 15 negotiated procurements and does not apply to orders placed against federal supply schedule contracts (FSS) under FAR subpart 8.4. The reason for that is FAR 8.404, Use of Federal Supply Schedule, which states in paragraph (a), "parts 13 (except 13.303-2(c)(3)), 14, 15, and 19 (except for the requirements at 19.102(b)(3) and 19.202-1(e)(1)(iii)) do not apply to BPAs or orders placed against Federal Supply Schedules contracts (but see 8.405-5)."
I recommend contacting the GSA contracting officer responsible for the FSS contracts being ordered against to request guidance on the contract procedures for the acquisition of data. Remember FSS is for the purchase of commercial products and services. For commercial to the maximum extent practicable, terms and conditions in the contract should mirror customary commercial practices unless government unique terms are required by statute or executive order (See FAR 12.301(c)). Finally, I strongly recommend discussing strategies for purchasing the technical data that is needed for your requirement with your agency contracting officer, your agency legal advisor and the GSA contracting officer.