I found this question, but I'm still wondering if there were a way that a router could forward a laptop's MAC address to the modem, and if the modem could forward it to the ISP. Are there routers and modems that are modified to do this, such as the ones that ISPs give to their customers?
Modems are bridges and work at the link layer (where Ethernet runs). They can vary a bit depending on the WAN technology they work with. But if the WAN connection deals with Ethernet frames (such as VDSL, GPON, DOCSIS, or ADSL IPoE) then the entire frame, including MAC addresess, is preserved. Whatever MAC address the modem sees, the ISP also sees.
Routers work at the network layer (where IP runs). They do not preserve the lower layer headers – all MAC addresses are stripped on input, and a new Ethernet frame is generated on output. From the modem's and the ISP's point of view, everything is sent from the router's own MAC address.
Routers (like hosts) can be configured to use any MAC address for their interfaces. If your ISP or modem expects a specific source MAC, you can easily configure that on your router's "WAN" interface. This is often called "MAC address cloning" in the router's settings screen.
However, even if you use MAC cloning, a router will still use only one MAC address per interface – it will not dynamically preserve the MAC addresses of all your internal hosts, and I don't think I've seen a configuration that would do so while still preserving the routing function.
The only time ISP-issued devices preserve MAC addresses of every LAN host is when they're configured as bridges, not routers. For example, when you have a multifunction modem, it'll use its own MAC address if routing is enabled, but it'll preserve MAC addreses if bridging is enabled.