G'Day Sally and welcome to the Forum. Ah bed bugs - well if you even have the slightest suspicion that you have the little critters you MUST advise the hospitalero at the first albergue you come too. So they can help you get rid of them. Similarly if you think you might have them when you arrive in Santiago I strongly recommend you thoroughly was everything (including the backpack itself); you sleeping bag can go into a drier - at high heat to kill the bed bugs and their eggs. When you get home - leave just about everything outside - in hot sun if possible, then go through the cleaning process again.
As for carrying your backpack as carry on luggage it will depend upon how large it is. Here I will make an assumption that you are flying Emirates out of Madrid - so an an economy class passenger you have a 7kg weight limit; business and first class have better carry-on allowances however your backpack will still need to be able to fit into the overhead luggage lockers/bins.
Like C clearly the airline might not have restrictions however I strongly doubt they will be happy to carry your infected bag as cabin luggage - nor will any other fellow passenger whose carrying on bags may come in contact with your bag and also become infected.
If you do some searches you will find a fair amount of info on how to avoid the buggers and how to get rid of them. One other thing - you should spray your Camino gear - backpack/sleeping bag/liner - (inside and out) with Permythian (sic?) - I think you get in from Chemists, but I will check (if someone else does not provide advice).
Should you have any more questions please don't hesitate to come back here. If you live in Sydney (or nearby) with have a gathering of Camino nuts each month. Have a look on the Australia/New Zealand page (towards the bottom of the home page). Cheers & Buen Camino.
