Or, hah, this probably works:

diff --git a/nixos/lib/systemd-lib.nix b/nixos/lib/systemd-lib.nix
index 3129fbe7bdb..4ee40f2fa08 100644
--- a/nixos/lib/systemd-lib.nix
+++ b/nixos/lib/systemd-lib.nix
@@ -332,7 +332,7 @@ in rec {
     systemd
   ];
 
-  initrdServiceConfig = mkServiceConfig [];
+  initrdServiceConfig = mkServiceConfig ["/"];
 
   mountConfig = { config, ... }: {
     config = {

okay so about the PATH stuff: I came up with this:

diff --git a/nixos/lib/systemd-lib.nix b/nixos/lib/systemd-lib.nix
index 3129fbe7bdb..7850d5f4524 100644
--- a/nixos/lib/systemd-lib.nix
+++ b/nixos/lib/systemd-lib.nix
@@ -396,6 +396,12 @@ in rec {
       text = commonUnitText def +
         ''
           [Service]
+          ${concatMapStrings (n:
+              let s = optionalString (def.environment.${n} != null && n == "PATH" -> def.environment.${n} != ":")
+                (lib.traceValSeq "Environment=${builtins.toJSON "${n}=${def.environment.${n}}"}\n");
+              # systemd max line length is now 1MiB
+              # https://github.com/systemd/systemd/commit/e6dde451a51dc5aaa7f4d98d39b8fe735f73d2af
+              in if stringLength s >= 1048576 then throw "The value of the environment variable ‘${n}’ in systemd service ‘${name}.service’ is too long." else s) (attrNames def.environment)}
           ${attrsToSection def.serviceConfig}
         '';
     };