If you have previously converted to the travel package and have the certificate in your MR account, doesn't the option exist to attach the certificate at the time you are reserving online? This is how it works when we make a reservation and want to use our free night CC certificate.
There is a way around having a points reservation that you really need not take all your points. You first make a dummy reservation that uses most of your MR points. Then you make the real reservation that you do want. You then cancel the original reservation and the points are then returned to your account. You can then attach a certificate to the real reservation by calling Marriott Rewards customer service. This is very useful for us and we have used it when we don't yet have enough points for a travel package. So we make the dummy reservation then the real one followed by cancelling the dummy. Then when we have enough points for the travel package, we convert and have them attach the certificate to the points reservation.
For the 1-night freebie that is given for the Marriott Visa, you can use that to attach to an online reward booking. For travel packages, there is not a way to attach it.
Back in Nov, I did not have enough for both a Cat 8 and Cat 9 so I booked two Cat 1-5 5-night travel packages, wanting to use them in Dec 2018 in Singapore (Cat 8) and Hong Kong (Cat 9). I needed the United Airlines points plus wanting to get their 25K bonus.
Anyway, I got the first Cat 1-5 hotel portion upgraded to Cat 8 for Singapore a couple of weeks ago. The math for that, they deducted 75K, and refunded 10K, net increase of 65K which is consistent, i.e., upgrading from 235K Cat 1-5 to 300K for Cat 8. Why I am breaking this down... read below for the Cat 9 upgrade.
BTW, I did think about using your workaround in booking and cancelling because I do now have alot of MRPs in my account, great minds

!!!
Since I will need to call Marriott to upgrade my 2nd Cat 1-5 cert to Cat 9 anyway, I have to make that 1 call to Marriott Rewards desk first. The 2nd call will be to attach the certificate so I might as well have Reservation desk make the reservation and attach the certificate at the same time. Here is what happened today with regards to upgrading the package. On the chart, Cat 9 5-night travel package requires 345K points. It should require 110K points to upgrade from Cat 1-5 to Cat 9, right???? The agent repeated several times that the upgrade would take 95K points. I told her it should be 110K but I was not going to argue with her. When it was completed, it showed that it took 105K and refunded 10K, net of 95K. So Cat 9 5 nights should really require 330K. What am I missing here???? If there is such a discrepancy, I will book Cat 1-5 travel package first before upgrading to Cat 9 in the future. In the past I just booked a straight Cat 9 travel package.
The Hong Kong stay date has not opened up yet and when it does I will call the Marriott Platinum Reservation desk to book the dates and attach the certificate.