I can't see it happening. There are several issues with this idea:
For a start it breaks Melbourne's squad cap as they then have to accommodate him as a full-time player even if he's only there for a third of a season, meaning they miss out on signing a foreign marquee. Considering that the A-League only allows one foreign "DP" (youth marquee slot aside) this would essentially put big question marks over their side for the rest of the season, it would be like NYCFC only offering its 3 DPs 6 month contracts.
Secondly, it gives Lampard a get-out clause. You can bet that he would be absolutely bombarded with offers from Europe, the Middle East and perhaps even MLS trying to persuade him to renege on his NYCFC deal, and if any of them succeeded in turning his head, there would be absolutely nothing the club could do to stop him simply flying off to some other city after his six months in Melbourne were up.
Thirdly, it's not certain what the rules are with the A-League exactly. You're only allowed one guest player, and you're only allowed one loanee. It seems like no-one really knows whether loanees count as guests, though. Simply put, it doesn't really happen much. Also, in the past clubs (read: Sydney FC) have managed to use accounting tricks to circumvent the cap on guests anyway, so some people are suggesting that the 1 guest and/or loan limit is worth about as much as the paper it's written on anyway.