Other extracts that would support this position are:
In the Ceremonial of Bishops it has, in the Chapter “Stational Mass of the Diocesan Bishop”.
“165. … Another deacon or one of the concelebrants takes any remaining consecrated particles to the tabernacle …”.
From the GIRM:
“317. In no way should all the other things prescribed by law concerning the reservation of the Most Holy Eucharist be forgotten. [footnote 131: Cf. particularly in Sacred Congregation for the Sacraments, Instruction Nullo umquam tempore, 28 May 1938: AAS 30 (1938), pp. 198-207; Codex Iuris Canonici, cann. 934-944.].”
A translation of this 1938 document is at
RomanRite - Reservation of the Eucharist . It includes:
“6. ( c) The key of the tabernacle must be most diligently kept by a priest. All the cautions mentioned up to the present will be in vain, if the chief caution, namely, the safe-keeping of the key of the tabernacle be neglected. The fourth paragraph of the above canon expressly mentions in respect to this point that a grave burden rests on the conscience of the priest to whom the key of the tabernacle is entrusted. In order to satisfy this obligation of most diligent custody in regard to the key, the Rector is solemnly warned that the key of the tabernacle must never be left on the table of the altar, nor in the door of the tabernacle, not even at the time when the divine offices are carried out in the morning at the altar of the Blessed Sacrament, or communion is distributed, especially if this altar is not in open view. When these offices are over, the key must be kept by the Rector at home, or always carried about by him, care being taken against losing it; or let it be kept in the sacristy in a safe and secret place, under lock and key, the second key being kept by the Rector as above.”
Counter arguments, supporting that the altar server can remove the consecrated hosts from the tabernacle, might be along these lines:
- None of the liturgical books describe how consecrated hosts are to be removed from the tabernacle during Mass.
- The requirements of the 1938 document about only a priest keeping the tabernacle key no longer apply, since Extraordinary Ministers of Holy Communion are now able to expose and repose the holy eucharist for the adoration of the faithful (as described in the liturgical book Holy Communion and the Worship of the Eucharist Outside of Mass, n. 91).
- Canon 938 has “The person in charge of a church or oratory is to see to it that the key of the tabernacle in which the blessed Eucharist is reserved, is in maximum safe keeping.” (1997, New revised English Translation, ISBN 000599375X). So conceivably the parish priest could decide that this “maximum safe keeping” is best achieved by the altar server holding the tabernacle key, rather than the Priest.