Can I have subledger accounting entries in Adjustment Only ledgers?

No, you can use Adjustment Only ledgers for General Ledger manual journals only. However, you can define a Subledger level secondary ledger with valuation method enabled and use it for subledger journal adjustments.

You can then create a ledger set to combine the secondary ledger with the primary ledger. Use the ledger set while running reports to obtain a complete accounting representation that includes both transactional data and adjustments.