MessageOps has released a nice script which you can use to notify your BPOS users when their Online Passwords are about to expire.

Hereby a copy of the script.
Original source here

#Microsoft Online Password Expiration Notification Script
#
#Written By:Chad Mosman, MessageOps, www.messageops.com
#
#This script notifies users via email when their Microsoft Online Password is about
#to expire. It is designed to be scheduled to run on a daily basis. Due to the way
#it searches for users, it requires directory synchronization be enabled for the domain
#it is run against.
#
#The following variables should be modified before running the script
#
#$AdvancedWarning – Controls how many days before expiration the users will be notified
#that their password is about to expire. Default is 15 days.
#
#$mailFrom – Enter the email address that the notification will appear to come from.
#
#$SMTPServer – If inbound mailflow is enabled for your Microsoft Online domain, the default of
#mail.global.frontbridge.com should work. Otherwise, specify the name of your on-premise
#mail system.
#
#$powerUser – Username of an account with Service Admin Rights in Microsoft Online.
#
#$powerpass – Password of the account with Service Admin Rights in Microsoft Online.
#
#$subject, $body – The notification message subject and body can be customized to your needs.
#
#When testing it is recommended the script be run against a single user. To do that, change:
#
#$collitems = Get-XsActiveDirectoryUser -Identity *
#To
#$collitems = Get-XsActiveDirectoryUser -Identity EmailAddressOfTestUser
#
#For assistance with the script, to report problems, or provide comments contact support@messageops.com
#
#Number of days in advance the user should be warned that their password is about to expire
$AdvancedWarning=15
#Email address that the notification email will appear to be from
$mailFrom = "user@yourdomain.com"
#If inbound mailflow is not enabled on your domain in Microsoft Online, change this value
#to your on-premise mail server which should forward to Microsoft Online
$smtpServer = "mail.global.frontbridge.com"
#Microsoft Online Service Account Username and Password
$powerUser = "user@domain.microsoftonline.com"
$powerPass = "Password"
$password = ConvertTo-SecureString $powerPass -AsPlainText -Force
$adminCredential = New-Object -TypeName System.Management.Automation.PSCredential -argumentlist $powerUser,$password
#Get all objects in your local Active Directory that are synchronized to Microsoft Online
$collitems = Get-XsActiveDirectoryUser -Identity * -Resultsize 100000 -quiet| Search-XsMicrosoftOnlineDirectory -Credential $adminCredential
foreach ($objitem in $collitems){
#Determine if the user has been activated or not
if($objitem.HardMatchName -ne $null){
$mailbox = get-xshostedExchangeMailbox -SourceIdentity $objitem.HardMatchName -sourceserver domain.com | Search-XsMicrosoftOnlineDirectory -credential $adminCredential
#check to see if the account is activated
if($mailbox.TargetSendQuota -gt 0){
#format the email address
$emailAddress = $mailbox.HardmatchName -replace "SMTP:",""
#get the password expiration date for the current user
$user=Get-msonlineuser -identity $emailAddress -credential $adminCredential
#calculate the date difference between today and the password expiration date
$datedifference=($user.PasswordExpirationDate-[DateTime]::Now).Days
#is the password going to expire withing the number of days configured in the AdvancedWarning?
If ($datedifference -le $AdvancedWarning){
If ($datedifference -eq 0){
$subject = "IMMEDIATE ACTION REQUIRED: Your Microsoft Online Password Has Expired"
$body = "Your Microsoft Online password has expired. "
}
ElseIf ($dateDifference -eq 1){
$subject = "IMMEDIATE ACTION REQUIRED: Your Microsoft Online Password will expire in 1 day"
$body = "Your Microsoft Online password will expire in 1 day. "
}
Else{
$subject = "ACTION REQUIRED: Your Microsoft Online Password will expire in",$datedifference,"days"
$body = "Your Microsoft Online password will expire in",$datedifference,"days. "
}
$body = $body + "Please use the Microsoft Online Sign in Client to change your password. If you do not use the Sign In Client, browse to https://home.microsoftonline.com to reset your password."
#send notification to user
$smtp = new-object Net.Mail.SmtpClient($smtpServer)
$smtp.Send($mailFrom, $emailaddress, $subject, $body)
}
}
}
}