I don't see gain/overloading as the biggest issue here. Head amps generally add 20-30 dB of voltage gain, and there are many line stages that add 20dB and a few more than that. The real issue is proper resistance loading of the cartridge. While you could use Y connectors to add a loading resistor, it would be in parallel with the input impedance of the preamp. This may or may not be a known quantity, and might vary greatly with frequency. Real head amps are designed to minimize these issues.
As long as you keep the volume down on the initial run, I doubt you can hurt anything by trying. If you've overloaded at any point in the chain it will become obvious in short order.
As long as you keep the volume down on the initial run, I doubt you can hurt anything by trying. If you've overloaded at any point in the chain it will become obvious in short order.