Answer
This has been identified as a genuine bug in Navigation Filter validation (confirmed on at least one release) rather than a sign the extra condition itself is unsupported — editing the link's underlying key-value pairs directly (rather than going through whatever UI step triggers the failing validation) has worked around it in practice, so if you hit a validation failure adding a second filter condition to a navigation link, try that direct key-value-pair edit before concluding the extra condition can't work at all.