May a Gabbai announce Yaaleh V’yavo or other important notices during davening ?
(1) Chazal state that one who had sinned could not participate in war even if he had only spoken between Yishtabach and Yotzer Ohr. The Alter Rebbe explains that Psukei D’zimra serves as introductory praise for Hashem and one should not interrupt between it and the rest of davening . While Shulchan Aruch records an allowance to break after Yishtabach (before Kaddish and Borchu ) for a Mitzva purpose , Minhag limits it to a pressing Mitzva and not one that can be performed later . Thus, it is not appropriate to mouth learning or recite Tehillim at that point.
(2) During Psukei D’zimra, one may not interrupt even for a pressing Mitzva, since the Brachos of Boruch She’amar and Yishtabach must link to each other . Yet, some Poskim distinguish between other Mitzvos and interrupting for the sake of davening itself. The interruption should be made after Boruch Hashem L’olam to conclude Boruch She’amar with a Bracha, and then repeat Boruch Hashem L’olam to begin Yishtabach with a Bracha .
(3) During Birchos Krias Shema and before Shmoneh Esrei of Shacharis – when one must adjoin Geulah (Go’al Yisroel) and Tefilah – it is forbidden to interrupt even for the sake of davening. Some are lenient at Maariv when adjoining Geulah and Tefilah is voluntary . However, Minhag Chabad is not to interrupt at Maariv either . Instead, an appointed individual should bang to signal an addition or read aloud several words of the addition during Shmoneh Esrei .
What about announcing page numbers?
Some consider this a necessity in certain circumstances and permissible after Yishtabach  or during Psukei D’zimra . If pages must be announced during the sections where interruptions are forbidden, the Gabbai may daven earlier so that he can announce the pages. However, Lechatchila one should avoid this Shaila by posting the relevant page numbers where all can see.
. .שוע’ר סי’ נ’א ס’ד
. .שוע’ר סי’ נ’ד ס’ג
. .ראה שוע’ר או’ח סי’ נ’א ס’ד (ועיי’ש שהמפסיק חוזר מעורכי המלחמה)
. .ערוה’ש סי’ נ’ד ס’ד, כף החיים סי’ נ’ד סקי’א
. .שוע’ר שם
. .שוע’ר שם ס’ה
. .או’ח סי’ רל’ו מ’ב סק’ז ושעה’צ סק’ז
. .ראה קצוה’ש סי’ כ’ז ס’ה ע’פ הסידור
. .כף החיים סי’ רל’ו סקט’ז (בשם רש’ל)
. .אבל יש לציין שבאם החזן הוא זה שמפסיק הרי יצטרך להגיד פסוקים מפסוקד’ז לפני אמירת קדיש – שוע’ר סי’ נ’ד ס’ד
. .שו’ת אג’מ ח’א סי’ כ’ב