For signing only, a 512-bit RSA key ought to resist at least a few days, more probably a few weeks, even against determined attackers. This is still "reasonable" as long as you verify the signature "soon". You can imagine that from the point the public key was made public, you have a few minutes, at best hours of security, after which you must consider the key as expired and refuse to trust signatures computed with that key.
I find your use case a bit dubious, though. Generating the RSA keys will be quite expensive with regards to computing a signature. If you worry about signature size, then you should investigate DSA or its elliptic curve variants. ECDSA with a 160-bit curve will use 320-bit signatures (quite smaller than 512-bit signatures for RSA-512) while being substantially stronger (ECDSA with a 160-bit curve is still beyond the technologically feasible) and will still be competitive with regards to CPU usage.
Also, the hard part of asymmetric cryptography is key distribution (that's what PKI is about); if you generate many keys, you only make that part harder.