If customary, I think an organ prelude before the Mass could be justified.
The playing of the organ or harmonium only is permitted . . . from the beginning of the solemn evening Mass in Cena Domini [emphasis mine] until the end of the Gloria in excelsis Deo
It must be acknowledged that this technically treats the time after the Gloria is "said". I will also grant that when considered in light of GIRM 313 there is slight wiggle room:The Gloria in excelsis is said. While the hymn is being sung, bells are rung, and when it is finished, they remain silent until the Gloria in excelsis of the Easter Vigil, unless, if appropriate, the Diocesan Bishop has decided otherwise. Likewise, during this same period, the organ and other musical instruments may be used only so as to support the singing.
...In lent the playing of the organ and musical instruments is allowed only in order to support the singing. Exceptions, however, are Lætare Sunday, Solemnities, and Feasts.
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