Hi,
I don't think that PI provides capabilities of setting allowed cipher suites per communication channel. Hence you can try to do this outside of SAP which brings additional complexity to your landscape. You could introduce a reverse proxy that would have hostnames like customer1.weakssl.local, customer2.weakssl.local for every customer that still needs SSLv3. This proxy would accept only TLS connection so you would be able to set ssl/client_ciphersuites to allow only strong suites. Your PI system would connect to partners with TLS or this reverse proxy only. The reverse proxy would drop TLS connection from PI and establish new connection SSLv3 between itself and customer. Hence PI would never use SSLv3 and the allowed SSLv3 outbound connections would be controlled by reverse proxy configuration.
PI ------------TLS----------------> Client that supports TLS
PI ------------TLS----->Reverse Proxy -----------SSLv3 ---------> Client that does not support TLS
Another disadvantage is that you will have to update PI config to connect via new reverse proxy instead of direct connection to customer's system.
Cheers