It says in SA OC Siman 629 Sif 3 that one may use for scach "chitzim" that don'e have a beis kibbol. However if they do have a beis kibbul then it's pasul for scach. The M"A writes in Sif-Katan Beis there that kanim that have a beis kibbol they are kosher since they weren't made to be "m'kabel". Seemingly this is similar to our bamboo sticks or the like that people use for scach. However what would be the din of one would put some sort of "chomer" (a poision or the like) used to kill bugs inside of the sticks. I'm not talking about a spray or something of the like rather something that has to it mamoshos (substance) that sits in the sticks and helps to kill the bugs. Even though these kanim like the M"A says weren't made to be a beis kibbol however now l'maseh being used it such a way does this pasul it for being scach? Perhaps there is a re'ah from Seder Taharas to this or another place in Shas and Rishonim in terms of what makes a cli into a cli, etc.
Yehoshua, Yerushalayim, Eretz Yisrael
1. The question here could possibly be phrased as whether "Yichud" (designating a utensil for a specific use) applies to making Kelim which are not "Na'asu l'Kabalah" (this is the term used in Mikva'os 4:3, which is the source of the Din of the Magen Avraham that you cited, to indicate that something was not made with the intention that it should contain items) into Kelim that are "Na'asu l'Kabalah."
2. It seems that proof that Yichud achieves this status for the Kli can be found in Kidushin (end of 59a), where the Gemara cites a Mishnah that "Kol ha'Kelim Yordin li'Yedei Tum'asan b'Machshavah" -- all utensils can be made fit to receive Tum'ah due to the "thought" or intention of the owners. If the "Yichud" when the owners decide to use it as a Kli transforms it into a Kli which can receive Tum'ah, then it seems logical that the Yichud when the owners decide to place things inside the Kli, even though it was not made for things to be placed inside it, also can make the Kli capable of receiving Tum'ah.
3. The above reasoning also appears to be the source of the Aruch ha'Shulchan (OC 629) who, when citing the above Din of the Magen Avraham, adds a few crucial words. He writes that for the reeds to be Kosher we require also that they be "Ein Omdin l'Kabalah" -- they are not going to be used to hold items. In other words, it is not sufficient that the reeds were not made for Kabalah; for them to be valid as Sechach they also must not be used in practice for holding items. Accordingly, if the bamboo sticks are designated to have poison placed inside them, it would seem that they are not valid for Sechach.
4. However, it seems that a few practical questions remain which must be asked before this matter can be settled. Why is the poison placed inside the reeds? Would it not stick just as well on the outside? In addition, for how long a period of time does one intend to use the reeds to place the poison inside them? This may depend on the question of whether Yichud l'Olam is required (the Kli must be designated for this use permanently) or whether Yichud l'Sha'ah suffices (one will use the reeds in such a way only for a short time)?
Kol Tuv,
Dovid Bloom