SAP validates that our delivered applications run with the version of SAPUI5 which is delivered with HANA. We update the UI5 version generally only with the SPS releases because a change in the version requires re-validation of all standard applications. So the rule is if you want a newer version of UI5, you should update to the latest SPS of HANA.
That said, it would be possible to install a newer version of UI5 in a separate package structure (not sap.ui5.1). This way the standard applications run against the standard version of UI5 for your revision and your custom applications could be pointed to a different version. However you would then take responsibility to always manually update this other version of SAPUI5. No revision or SPS update would ever touch it.