Statutory declarations are not just used for YOUR intended purpose. You may be using it for a LTSVP but it can be used as evidence of the matters stated in the Stat Dec. So yes, the Stat Dec can be used for property hearings for a common law marriage in jurisdictions where they are recognised - even though this was not your reason or intent for signing the document.
Non 'certified' marriages are not recognised in Singapore and I suspect, but do not know, that as Greece is largely a catholic orthodox country, that they may not hold much water in Greece either.
On the other hand if either of you have assets and some connection to a country that does recognise 'de-facto' relationships (eg Australia, UK, USA etc), either of you can pursue the other for a share in those assets. I believe you said you were a US citizen. Therefore, if you hold property in US - be aware that you have granted your other half an 'interest' in that property.
But though all this I am assuming you are not a gay couple. If you are, it is a whole other story in relation to assets of the relationship.
lol
