This is one of the many reasons I don't use Pages. If it was nicer to use and didn't have these sorts of feature-failure hangups I would probably use it, but alas...
Here's a basic UI-Script for doing what you want.
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# Auth: Christopher Stone
# dCre: 2017/05/05 23:25
# dMod: 2017/05/05 23:39
# Appl: Pages
# Task: Go to next bookmark (first bookmark if none are selected).
# Libs: None
# Osax: None
# Tags: @Applescript, @Script, @Pages, @System_Events, @Go-To, @Next, @Bookmark, @first, @None, @Selected
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tell application "System Events"
tell application process "Pages"
tell (first window whose subrole is "AXStandardWindow")
tell toolbar 1
tell radio group 1
tell radio button "Document"
if its value ≠ 1 then
click
end if
end tell
end tell
end tell
tell splitter group 1
tell radio group 1
tell radio button "Bookmarks"
if its value ≠ 1 then
click
end if
end tell
end tell
tell scroll area 2
tell table 1
set focused to true
set selRows to rows whose selected is true
if selRows = {} then
select row 1
else
try
set selRowIndex to value of attribute "AXIndex" of (first row whose selected is true)
select row (selRowIndex + 1 + 1)
end try
end if
end tell
end tell
end tell
end tell
end tell
end tell
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