You'll have to look at the RaidStatus property, which contains more than just the read-only flag. So you can use "-match" or "-notmatch" like this:
PS C:\> get-navol | ? {$_.RaidStatus -match "read-only"} | ft Name, State, RaidStatus
Name State RaidStatus
---- ----- ----------
rre1deleteme online raid_dp,snapmirrored,read-only
rresql online raid_dp,snapmirrored,read-only
Cheers!
Eric