I will give my opinion on the subject again:
Merseyrail is aware and looking at these "spare" 508s. However, I understand that they are being looked at for parts and spares rather than enhanced capacity.
The constant year for renewal of rolling stock being floated around depots/messrooms etc seems to be 2014. Whether there is any truth in this is questionable, although I do think it is a believable and realistic date, that's just over 4 years to source a replacement.
The last time I spoke to Bart (he is the big boss, obviously you know FC but others won't) he said that they are looking to obtain units from London Overground. Considering that London Overground only have three 508s, that would correlate with what you say about looking for spares rather than more capacity.
The current lease for the Merseyrail units is up in 2014 so it would be a natural time for replacements.