I tested this in a creative world with an HTGR. If the steam boilers are adjacent to the heat exchanger without any heat pipes, everything works as expected. Using heat pipes to connect the heat exchanger to the boilers causes almost all of the heat to be lost and little to no steam can be generated.
The heat pipes can still be heated by the exchanger and they'll retain their stored energy on their own, as long as they aren't connected to anything that can accept heat from them. If they are, then that stored heat will be quickly voided. This is easily visible when connecting heat pipes to a blast furnace.
I tested this in a creative world with an HTGR. If the steam boilers are adjacent to the heat exchanger without any heat pipes, everything works as expected. Using heat pipes to connect the heat exchanger to the boilers causes almost all of the heat to be lost and little to no steam can be generated.
The heat pipes can still be heated by the exchanger and they'll retain their stored energy on their own, as long as they aren't connected to anything that can accept heat from them. If they are, then that stored heat will be quickly voided. This is easily visible when connecting heat pipes to a blast furnace.
The heat pipes also do not glow.